Hydride transfer reactions of transition metal hydrides: Kinetic hydricity of metal carbonyl hydrides

J. Am. Chem. Soc. 1998, 120, 13121-13137

13121

Hydride Transfer Reactions of Transition Metal Hydrides:

Kinetic Hydricity of Metal Carbonyl Hydrides

Tan-Yun Cheng, Bruce S. Brunschwig, and R. Morris Bullock*

Contribution from the Chemistry Department, BrookhaVen National Laboratory,

Upton, New York 11973-5000

ReceiVed June 8, 1998

-

Abstract: Hydride transfer from neutral transition metal hydrides (MH) to Ph₃C⁺BF₄gives M-FBF₃and

Ph₃CH. The rate law -d[Ph₃C⁺BF₄^-]/dt ) k[Ph₃C⁺BF₄^-][MH] was established from kinetic measurements

using stopped-flow methods. Second-order rate constants determined in CH₂Cl₂solution at 25 °C range from

k_H) 7.2 × 10^-1M^-1s^-1to k_H) 4.6 × 10⁶M^-1s^-1. The order of increasing kinetic hydricity is (C₅H₄-

CO₂Me)(CO)₃WH < (CO)₅MnH < Cp*(CO)₃CrH < Cp(CO)₃WH < HSiEt₃< cis-(CO)₄(PCy₃)MnH < cis-

(CO)₄(PPh₃)MnH < (C₅H₄Me)(CO)₃WH < Cp(CO)₃MoH < Cp*(CO)₃WH < (indenyl)(CO)₃WH < (CO)₅ReH

< Cp*(CO)₃MoH < cis-(CO)₄(PPh₃)ReH < Cp(NO)₂WH < trans-Cp(CO)₂(PCy₃)MoH < trans-Cp(CO)₂-

(PPh₃)MoH < trans-Cp(CO)₂(PMe₃)MoH (Cp ) η⁵-C₅H₅, Cp* ) η⁵-C₅Me₅, Cy ) cyclohexyl). Ranges of

activation parameters for hydride transfer from trans-Cp(CO)₂(PMe₃)MoH, trans-Cp(CO)₂(PCy₃)MoH, cis-

-

(CO)₄(PPh₃)ReH, and Cp*(CO)₃MoH are ∆H^q) 3.0-5.9 kcal mol^-1and ∆S^q) -18 to -24 cal K^-1mol^-1

.

-

The rate constant for hydride transfer (k_H) from cis-Cp(CO)₂(PCy₃)MoH at -55 °C is 3 orders of magnitude

lower than that for trans-Cp(CO)₂(PCy₃)MoH. Phosphine substitution for CO generally enhances the kinetic

hydricity, with trans-Cp(CO)₂(PMe₃)MoH being 10⁴times as reactive as Cp(CO)₃MoH. The electronic effect

of phosphine substitution is attenuated by steric factors when the phosphine is cis to the metal hydride. The

hydride transfer kinetics reported here are interpreted to be single-step hydride transfers, rather than a multiple-

step mechanism involving an initial electron transfer followed by hydrogen atom transfer. A distinction is

made between hydricity and nucleophilicity of metal hydrides.

Introduction

hydridic reactivity akin to that of main-group hydrides. Labinger

and Komadina reported⁴qualitative rates of reduction of ketones

by a series of metallocene hydrides; presumably, the ketone

coordinates with the metal prior to hydride transfer in some or

all of these cases. From studies of the relative reactivity of these

metal hydrides with acetone and with (CF₃)(CH₃)CdO, Lab-

inger and Komadina determined that the relative order of

hydridic character was [Cp₂ZrH₂]_n> Cp₂NbH₃> Cp₂MoH₂>

Cp₂ReH (Cp ) η⁵-C₅H₅). An important conclusion drawn from

their work is that the hydridic character is highest for metals to

the left of the transition metal series, and falls off substantially

on moving to the right of the periodic table.

Anionic metal carbonyl hydrides exhibit pronounced hydridic

reactivity.⁵Darensbourg and co-workers have carried out

detailed investigations on the reactions of (CO)₅WH^-and a

series of related anionic metal hydrides, and have demonstrated

the utility of these metal hydrides in the reduction of alkyl

halides,⁶acyl chlorides,⁷aldehydes⁸and ketones.⁸

Many homogeneous catalytic processes¹rely on transition

metal hydrides. Large-scale industrial reactions such as olefin

hydroformlyation, hydrocyanation, and hydrogenation all in-

volve metal hydrides, often in more than one step of the catalytic

cycle. In most cases the metal hydrides are regenerated in

catalytic reactions by reaction with H₂. Transition metal hydrides

are also valuable stoichiometric reagents for organic synthetic

reactions. Development of a better understanding of the details

of how the hydride ligand is delivered to an organic substrate

is a fundamentally important goal pertinent to a wide range of

catalytic reactions. The results of kinetic and mechanistic studies

interrogating processes of M-H bond cleavage can provide

guidance in the rational design of new catalytic reactions.

The relevance of hydride transfers from metal hydrides to

bio-inorganic chemistry has also been documented. Hembre and

McQueen have recently demonstrated Ru-catalyzed hydride

transfers to NAD⁺model compounds that are relevant to

hydrogenase enzymes.²In bio-organic chemistry related to

nicotinamide coenzymes, many kinetics studies have examined

hydride transfers from substituted dihydropyridines to pyri-

dinium (and related heteroaromatic cations).³

(3) (a) Bunting, J. W. Bioorg. Chem. 1991, 19, 456-491. (b) Kreevoy,

M. M.; Kotchevar, A. T. J. Am. Chem. Soc. 1990, 112, 3579-3583. (c)

Kreevoy, M. M.; Ostovic, D.; Lee, I.-S. H.; Binder, D. A.; King, G. W. J.

Am. Chem. Soc. 1988, 110, 524-530. (d) Lee, I.-S. H.; Ostovic, D.;

Kreevoy, M. J. Am. Chem. Soc. 1988, 110, 3989-3993.

(4) Labinger, J. A.; Komadina, K. H. J. Organomet. Chem. 1978, 155,

C25-C28.

Main-group metal hydrides such as LiAlH₄and NaBH₄are

immensely useful for reduction of several types of organic

functional groups. Some transition metal hydrides exhibit

(5) For a review of anionic metal hydrides, see Darensbourg, M. Y.;

Ash, C. E. AdV. Organomet. Chem. 1987, 27, 1-50.

(6) Kao, S. C.; Spillett, C. T.; Ash, C.; Lusk, R.; Park, Y. K.;

Darensbourg, M. Y. Organometallics 1985, 4, 83-91.

(7) Kao, S. C.; Gaus, P. L.; Youngdahl, K.; Darensbourg, M. Y.

Organometallics 1984, 3, 1601-1603.

(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; John

Wiley & Sons: New York, 1992.

(2) (a) Hembre, R. T.; McQueen, S. J. Am. Chem. Soc. 1994, 116, 2141-

2142. (b) Hembre, R. T.; McQueen, J. S. Angew. Chem., Int. Ed. Engl.

1997, 36, 65-67.

(8) Gaus, P. L.; Kao, S. C.; Youngdahl, K.; Darensbourg, M. Y. J. Am.

Chem. Soc. 1985, 107, 2428-2434.

Published on Web 12/04/1998

13122 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

Scheme 1

(2)

-

Ph₃C⁺BF₄proceeds as shown in eq 2, producing Ph₃CH as

the organic product. Formal removal of H^-from an 18-electron

metal hydride would produce a 16-electron metal cation M⁺;

the actual isolated product contains coordinated BF₄^-. Several

of the M-FBF₃complexes produced in our kinetics studies have

been previously synthesized and characterized. Particularly

notable is the pioneering work by Beck in synthesis and

characterization of a series of M-FBF₃and related complexes.¹⁷

Each of the M-FBF₃complexes produced in the kinetic studies

in this paper were prepared and characterized to verify the

identity of the products and to establish that the reactions were

suitable for kinetic studies.

Neutral metal carbonyl hydrides exhibit diverse modes of

reactivity. As shown in Scheme 1, formal cleavage of the M-H

bond can generate a hydride, a hydrogen atom, or a proton.

Some neutral metal carbonyl hydrides (e.g., CpW(CO)₃H) can

undergo each of these three formal modes of M-H bond

rupture, in reactions with different substrates.⁹Detailed informa-

tion is available on the kinetic and thermodynamic acidity of

these neutral metal hydrides.¹⁰Relative rates of hydrogen atom

transfer from metal hydrides are known for their exothermic

reaction with carbon-centered radicals^11,12and for their endo-

thermic reaction with a substituted styrene.¹¹There is thus far

more quantitative kinetic information available on proton

transfer and hydrogen atom transfer reactions of neutral metal

carbonyl hydrides than on their hydride transfer reactions.

In this laboratory, we have shown that hydride transfer

reactions of neutral metal hydrides constitute the product-

forming step of ionic hydrogenations.¹³For example, ionic

hydrogenation of tetramethylethylene by CF₃SO₃H and CpW-

(CO)₃H is shown in eq 1; the mechanism involves hydride

Products other than M-FBF₃were observed for only three

hydride donors reported here: Cp*Cr(CO)₃H, Cp(NO)₂WH, and

-

HSiEt₃. Hydride transfer from Cp*Cr(CO)₃H to Ph₃C⁺BF₄

gave one equivalent of Ph₃CH, as shown by NMR, but the

organometallic product was not identified. The presumed initial

organometallic product, Cp*(CO)₃CrFBF₃, was not observed.

Apparently the hydride transfer reaction occurs cleanly, but the

initial product, Cp*(CO)₃CrFBF₃, is thermally unstable. This

is corroborated by earlier work,¹⁸which had shown that a

thermally stable derivative, [Cp*Cr(CO)₃P(OMe)₃]⁺BF₄^-, could

-

be prepared by reaction of Cp*Cr(CO)₃H with Ph₃C⁺BF₄at

low temperature, followed by addition of P(OMe)₃. The Cr

complex CpCr(CO)₃(NCMe)⁺BF₄was also found to be

-

unstable in CH₂Cl₂solution,¹⁹so the lower thermal stability of

Cr complexes of this type, compared with that of analogous

Mo and W compounds, appears to have some generality.

Hydride transfer from Cp(NO)₂WH to Ph₃C⁺BF₄produced

-

{[CpW(NO)₂]₂(µ-H)}⁺BF₄^-. This bimetallic hydride complex

was previously prepared²⁰by reaction of Ph₃C⁺BF₄^-with two

equivalents of Cp(NO)₂WH. Similar hydride-bridged bimetallic

complexes, [M(µ-H)M]⁺, are formed from other combinations

of MH + M-FBF₃. For example, Beck and Schloter prepared

{[CpW(CO)₃]₂(µ-H)}⁺BF₄^-from reaction of CpW(CO)₃H with

CpW(CO)₃FBF₃,²¹but under the conditions of our kinetics

experiments, the product was CpW(CO)₃FBF₃. Reaction of the

transfer from the metal to the carbenium ion formed through

protonation of the olefin. Although several metal hydrides [Cp-

(CO)₃WH, Cp*(CO)₃WH, Cp(CO)₃MoH, (CO)₅MnH, (CO)₅ReH,

and Cp*(CO)₂OsH; Cp* ) η⁵-C₅Me₅] can be used as hydride

donors in these ionic hydrogenations of olefins, this system was

not suitable for a detailed evaluation of kinetic hydricity, in

part because of complications from competing protonation of

the metal hydride by acid.¹⁴Although the importance of hydride

transfers from metal hydrides is clear, “the factors controlling

the hydricity are far from being well-understood.”¹⁵This paper

reports a study of the kinetics of hydride transfer from a series

of metal hydrides to Ph₃C⁺, in an effort to evaluate the factors

influencing kinetic hydricity.¹⁶

-

main group hydride donor HSiEt₃with Ph₃C⁺BF₄produced

FSiEt₃, which is in agreement with an earlier report.²²

Kinetic Measurements of Hydride Transfer. The kinetics

of hydride transfer (eq 2) were determined in CH₂Cl₂solution

by measuring the rate of disappearance of absorbance of

-

Ph₃C⁺BF₄at 450 nm (ꢀ ) 2.5 × 10⁴M^-1cm^-1). The

absorbance at 450 nm is at the shoulder of an absorption band

for Ph₃C⁺BF₄^-, λ_max) 410 and 430 nm, ꢀ ) 3.2 × 10⁴M^-1

cm^-1. Most of the kinetics were carried out by stopped-flow

techniques, but conventional UV-vis spectrophotometry was

used for monitoring the slowest reactions. These experiments

were carried out by using an excess of metal hydride, with

[MH]₀typically 10-100 times greater than [Ph₃C⁺BF₄^-]₀. Since

Ph₃C⁺reacts rapidly with water, we initially questioned whether

the solvent could be dried sufficiently to permit handling of

low concentrations of Ph₃C⁺. Fortunately, by using CH₂Cl₂

distilled from P₂O₅and handled in a drybox, initial concentra-

Results

Formation of M-FBF₃by Hydride Transfer from MH

to Ph₃C⁺BF₄^-. Hydride transfer from metal hydrides to

(9) For a review comparing hydride, proton, and hydrogen atom transfer

reactions of metal hydrides, see: Bullock, R. M. Comments Inorg. Chem.

1991, 12, 1-33.

(10) For a review of proton transfer reactions of metal hydrides, see:

Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides; Dedieu,

A., Ed.; VCH: New York, 1992; Chapter 9, pp 309-359.

(11) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112, 6886-

6898.

(12) Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R. J.

Am. Chem. Soc. 1991, 113, 4888-4895.

(13) Bullock, R. M.; Song, J.-S. J. Am. Chem. Soc. 1994, 116, 8602-

8612.

(17) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405-1421.

(18) Leoni, P.; Landi, A.; Pasquali, M. J. Organomet. Chem. 1987, 321,

365-369.

(19) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990,

112, 2618-2626.

(20) Hames, B. W.; Legzdins, P. Organometallics 1982, 1, 116-124.

(21) Beck, W.; Schloter, K. Z. Naturforsch. 1978, 33b, 1214-1222.

(22) Chojnowski, J.; Fortuniak, W.; Stanczyk, W. J. Am. Chem. Soc.

1987, 109, 7776-7781.

(14) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996,

15, 2504-2516.

(15) Dedieu, A. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH:

New York, 1992; Chapter 11, pp 381-387.

(16) For a preliminary communication, see Cheng, T.-Y.; Bullock, R.

M. Organometallics 1995, 14, 4031-4033.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13123

Table 1. Rate Constants for Hydride Transfer from Metal

Hydrides to Ph₃C⁺BF₄(CH₂Cl₂, 25 °C)

-

-1 -1

a

-

metal hydride

k_H(M

)

s

(C₅H₄CO₂Me)(CO)₃WH

(CO)₅MnH

Cp*(CO)₃CrH

Cp(CO)₃WH

HSiEt₃

cis-(CO)₄(PCy₃)MnH

cis-(CO)₄(PPh₃)MnH

(C₅H₄Me)(CO)₃WH

Cp(CO)₃MoH

Cp*(CO)₃WH

(Indenyl)(CO)₃WH

(CO)₅ReH

Cp*(CO)₃MoH

cis-(CO)₄(PPh₃)ReH

Cp(NO)₂WH

trans-Cp(CO)₂(PCy₃)MoH

trans-Cp(CO)₂(PPh₃)MoH

trans-Cp(CO)₂(PMe₃)MoH

7.2 × 10^-1

5.0 × 10¹

5.7 × 10¹

7.6 × 10¹

1.5 × 10²

1.7 × 10²

2.3 × 10²

2.5 × 10²

3.8 × 10²

1.9 × 10³

2.0 × 10³

6.5 × 10³

1.2 × 10⁴

1.9 × 10⁴

4.3 × 10⁵

5.7 × 10⁵

4.6 × 10⁶

Figure 1. Plots of k_obsvs [Cp*(CO)₃MoH] for data at -20 °C (0),

-10 °C (2), 0 °C ([), 10 °C (9), and 25 °C (B).

^a<(10% estimated uncertainty for rate constants.

tions of [Ph₃C⁺BF₄^-]₀in the range of 0.05 mM could be used.

Attempts to use CH₃CN solvent were thwarted by our inability

to dry it sufficiently to permit handling of CH₃CN solutions of

[Ph₃C⁺BF₄^-] at low concentrations. NMR experiments in CD₃-

CN were feasible, however, since higher concentrations of

Ph₃C⁺BF₄were used.

-

A typical plot of the absorbance vs time for a stopped-flow

experiment is provided in the supporting information (Figure

S1). Plots of ln(A_t- A_∞) vs time are linear, in some cases for

>8 half-lives, indicating remarkably clean pseudo-first-order

reactions (see Figure S2 in the supporting information). The

observed rate constants (k_obs) were obtained from nonlinear least

squares fitting of A_tto ∆A exp(-k_obst) + A_∞, where ∆A ) A₀

- A_∞. Plots of k_obsvs [MH] are also linear (see Figures S3-S8

in the supporting information). These data establish the second-

order rate law: -d[Ph₃C⁺BF₄^-]/dt ) k[Ph₃C⁺BF₄^-][MH].

Table 1 gives the second-order rate constants determined for a

series of metal hydrides; the hydrides are listed in order of

increasing hydricity. A kinetic isotope effect (k_MoH/k_MoD) 1.8;

Figure S7 in the supporting information) was determined by

comparing the rate constant for reaction of the metal hydride

Cp(CO)₃MoH with that for the metal deuteride Cp(CO)₃MoD.²³

In contrast to the large number of metal carbonyl hydrides

with cyclopentadienyl ligands, the examples of metal carbonyl

hydrides with arene ligands are far fewer. The manganese

hydride (η⁶-C₆Me₆)(CO)₂MnH was prepared by Eyman and co-

workers.²⁴Unfortunately, we found that (η⁶-C₆Me₆)(CO)₂MnH

decomposed slowly (∼10% in 1 h) in CH₂Cl₂to give free C₆-

Me₆and unidentified organometallic decomposition products.

Figure 2. Eyring plots for hydride transfer from trans-Cp(CO)₂(PCy₃)-

MoH (-20 to 25 °C) and cis-(CO)₄(PPh₃)ReH (-35 to 25 °C).

Cp(CO)₂(PCy₃)MoH (Cy ) cyclohexyl, C₆H₁₁), cis-(CO)₄-

(PPh₃)ReH, and Cp*(CO)₃MoH; activation parameters are listed

in Table 2. Figure 1 shows plots of k_obsvs [Cp*(CO)₃MoH] for

data at -20, -10, 0, 10, and 25 °C. Eyring plots of ln(k/T) vs

1/T are shown for trans-Cp(CO)₂(PCy₃)MoH and cis-(CO)₄-

(PPh₃)ReH in Figure 2. All of these reactions have low ∆H^q

values, ranging from 3 to 6 kcal mol^-1. The corresponding

entropies of activation (-18 to -24 cal K^-1mol^-1) are in the

range expected for bimolecular reactions.

Cis-Trans Interconversion of Cp(CO)₂(PR₃)MoH, and

Kinetic Hydricity of the Cis vs Trans Isomers. The phosphine-

substituted molybdenum compounds Cp(CO)₂(PR₃)MoH have

a “four-legged piano stool” structure and are known to exist as

an interconverting mixture of cis and trans isomers;²⁵K_eq

)

k_TC/k_CT(eq 3). Measurements on the kinetics and thermodynam-

Hydride transfer from (η⁶-C₆Me₆)(CO)₂MnH to Ph₃C⁺BF₄

-

(3)

does occur cleanly in CD₃CN solution, giving [(η⁶-C₆Me₆)(CO)₂-

Mn(CH₃CN)]⁺BF₄^-. A competition experiment was carried out

in which (η⁶-C₆Me₆)(CO)₂MnH and Cp*(CO)₃WH competed

as hydride donors to Ph₃C⁺BF₄^-in CD₃CN at room temperature.

From the ratio of the resulting [(η⁶-C₆Me₆)(CO)₂Mn(CH₃-

ics of cis h trans interconversion of the phosphine-substituted

complexes Cp(CO)₂(PR₃)MoH (R ) Me, Ph, Cy) were required

to interpret our kinetics data completely. The hydride and Cp

resonances of the cis and trans isomers of Cp(CO)₂(PCy₃)MoH

CN)]⁺BF₄and [Cp*(CO)₃W(CH₃CN)]⁺BF₄observed by

NMR, the relative kinetics of hydride transfer were ∼4 times

as fast for (η⁶-C₆Me₆)(CO)₂MnH as for Cp*(CO)₃WH.

The temperature dependence of the second-order rate con-

stants was determined for trans-Cp(CO)₂(PMe₃)MoH, trans-

-

1

are readily distinguished by H NMR at low temperatures. In

CD₂Cl₂at -86 °C, the hydride resonance for the cis isomer

appears at δ -6.25 and has a much larger J_PH(60 Hz) than that

found for the trans isomer (20 Hz) at δ -6.96.

(23) A complete discussion of isotope effects found for hydride transfer

from several metal hydrides will be reported separately: Cheng, T.-Y.;

Bullock, R. M., submitted for publication.

(24) Bernhardt, R. J.; Wilmoth, M. A.; Weers, J. J.; LaBrush, D. M.;

Eyman, D. P.; Huffman, J. C. 1986, 5, 883-888.

(25) Faller, J. W.; Anderson, A. S. J. Am. Chem. Soc. 1970, 92, 5852-

5860.

13124 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Table 2. Activation Parameters for Hydride Transfer from Metal Hydrides to Ph₃C⁺BF₄in CH₂Cl₂

Cheng et al.

-

metal hydride

∆H^q(kcal mol^-1

)

∆S^q(cal K^-1mol^-1

)

∆G^q(298 K) (kcal mol^-1

)

temp range (°C)

trans-Cp(CO)₂(PMe₃)MoH

trans-Cp(CO)₂(PCy₃)MoH

cis-(CO)₄(PPh₃)ReH

Cp*(CO)₃MoH

3.0 ( 0.1

4.5 ( 0.1

5.9 ( 0.1

5.0 ( 0.1

-18 ( 1

-20 ( 1

-24 ( 1

8.4

9.8

11.9

12.2

-55 to -25

-20 to +25

-35 to +25

-20 to +25

The equilibrium constant (K_eq) [cis]/[trans]; eq 3) for Cp-

(CO)₂(PCy₃)MoH exhibits a very small temperature dependence.

A value of K_eq) 8.1 (89% cis, 11% trans) was measured

directly by NMR at -86 °C. An almost identical value (91%

cis, 9% trans) was determined²⁵from the averaged J_PHat 22

°C; the cis and trans isomers are in the fast-exchange regime at

this temperature. For the PMe₃complex Cp(CO)₂(PMe₃)MoH,

the measured values ranged from K_eq) 0.81 at -86 °C to K_eq

) 1.1 at -1 °C. A van’t Hoff plot of ln K_eqvs 1/T for data

collected at eight temperatures over this range provided the

thermodynamic parameters ∆H° ) 0.34 ( 0.02 kcal mol^-1and

∆S° ) 1.4 ( 0.1 cal K^-1mol^-1; extrapolation to 25 °C gave

K_eq) 1.15 (54% cis; 46% trans). Similar values (∆H° ) 0.435

trans isomerization of Cp(CO)₂(PMe₃)MoH in toluene-d₈: ∆H^q

) 12.19 ( 0.07 kcal mol^-1and ∆S^q) -6.60 ( 0.25 cal K^-1

mol^-1

.

Since both cis and trans isomers are present in solution as an

interconverting mixture, a question arises about the relative rates

of hydride transfer from the two isomers. Tilset and co-workers

previously reported¹⁹that hydride transfer from Cp(CO)₂(PMe₃)-

-

WH to Ph₂(p-MeOC₆H₄)C⁺BF₄in CH₃CN gave [Cp(CO)₂-

(PMe₃)W(NCCH₃)]⁺BF₄as a 90:10 mixture of trans:cis

-

isomers. Isomerization of this mixture to the thermodynamic

isomer ratio of 5:95 trans:cis was very slow (t_1/2≈ 40 h).¹⁹

These authors suggested that the predominance of the trans

isomer as the kinetic product resulted from a selective reaction

of the trans hydride isomer with the substituted trityl cation.

Closely related experiments²⁷with the Mo hydride Cp(CO)₂-

(PPh₃)MoH similarly gave a predominance of the trans isomer

of [Cp(CO)₂(PPh₃)Mo(NCCH₃)]⁺. Since these results suggested

a higher reactivity of the trans isomers of phosphine-substituted

metal hydrides of this type, we decided to study the kinetics of

the reaction of Cp(CO)₂(PCy₃)MoH with Ph₃C⁺BF₄^-. This

hydride was selected for detailed examination since its cis isomer

predominates, providing a method of separately determining the

rate constants for hydride transfer from both its cis and trans

isomers.

( 0.005 kcal mol^-1and ∆S° ) 1.84 ( 0.02 cal K^-1mol^-1

)

were previously reported²⁶for Cp(CO)₂(PMe₃)MoH in toluene

solvent. For the PPh₃complex Cp(CO)₂(PPh₃)MoH in CD₂Cl₂,

we determined thermodynamic parameters of ∆H° ) 0.34 (

0.01 kcal mol^-1and ∆S° ) 2.1 ( 0.1 cal K^-1mol^-1from

measurements of K_eqat five temperatures between -86 and -24

°C. The extrapolated K_eq) 1.7 at 25 °C for our measurements

in CD₂Cl₂is the same as the K_eq) 1.7 reported by Faller and

Anderson²⁵for Cp(CO)₂(PPh₃)MoH in CDCl₃.

Although separate resonances for the cis and trans isomers

are well-resolved at low temperatures, line-broadening occurs

at higher temperatures because of interconversion of the cis and

trans isomers. Experimentally measured ¹H NMR line-broaden-

ing data for Cp(CO)₂(PCy₃)MoH at eight temperatures over the

range of -45 to 27 °C were analyzed by using a simulation

program to determine the rate constants (k_obs) k_TC+ k_CT) for

cis h trans interconversion. An Eyring plot is shown in the

supporting information (Figure S9); the activation parameters

for k_CTderived from these data are ∆H^q) 11.6 ( 0.3 kcal

We first consider the determination of the rate constant for

hydride transfer from the trans isomer, k_T, as shown in the

mechanism in Scheme 2. Experiments with a sufficient excess

Scheme 2

mol^-1and ∆S^q) -5.6 ( 1.2 cal K^-1mol^-1

.

Since the hydride transfer reaction involves conversion of

ionic Ph₃C⁺BF₄^-and neutral MH to neutral Ph₃CH and neutral

M-FBF₃, the ionic strength of the medium changes during the

hydride transfer reaction. To assess the effect of ionic strength,

we also determined the kinetics of cis h trans interconversion

of Cp(CO)₂(PCy₃)MoH in CD₂Cl₂in the presence of added

NBu₄⁺BF₄^-(0.11 M). Activation parameters determined for k_CT

from measurements at six temperatures between -35 and 27

°C were ∆H^q) 13.8 ( 0.2 kcal mol^-1and ∆S^q) 1.9 ( 0.7

cal K^-1mol^-1. The ionic strength (0.11 M) used in this

isomerization experiment is much higher than that used in any

of the hydride transfer kinetics experiments, which were carried

out at 13.5 mM for experiments with excess [Ph₃C⁺].

The metal hydride Cp(CO)₂(PCy₃)MoH is a new complex

prepared in this work, but several closely related Mo and W

compounds were reported previously. Faller and Anderson

reported a thorough study of the kinetics of cis h trans

isomerizations of a series of Cp(CO)₂(PR₃)MoX complexes,

where X ) H, halide, or alkyl.²⁵For cis f trans isomerization

of Cp(CO)₂(PⁿBu₃)MoH in CDCl₃, they reported ∆H^q) 11.7

( 0.1 kcal mol^-1and ∆S^q) -7.3 ( 0.4 cal K^-1mol^-1. Similar

values were found by Poilblanc and co-workers²⁶for the cis f

of metal hydride ([MH] . [Ph₃C⁺]) were carried out with

enough of the trans isomer present to completely consume the

Ph₃C⁺. Under these conditions, with k_TC. k_T[Ph₃C⁺] (i.e.,

the cis/trans equilibration is much faster than hydride transfer),

the observed rate constant is given by eq 4. (A more detailed

k_obs) k_T[trans-MH]

(4)

explanation is provided in the Appendix.) Thus k_Tis readily

determined from experiments with excess [MH] since [trans-

MH] is known. As indicated in Table 1, rate constants k_Twere

determined in this manner for all three of the phosphine-

substituted metal hydrides Cp(CO)₂(PR₃)MoH (R ) Me, Ph,

Cy).

The hydride transfer reactions have low values of ∆H^q(Table

2) so the rate constants for hydride transfer are only weakly

temperature-dependent. In contrast, the rate constants for cis

h trans isomerization of Cp(CO)₂(PCy₃)MoH exhibit a much

larger temperature dependence, so the isomerization rate slows

down more with decreasing temperature than does the hydride

transfer rate. Thus experiments using an excess of [Ph₃C⁺] were

(26) Kalck, P.; Pince, R.; Poilblanc, R.; Roussel, J. J. Organomet. Chem.

1970, 24, 445-452.

(27) Smith, K.-T.; Tilset, M. J. Organomet. Chem. 1992, 431, 55-64.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13125

carried out to determine the kinetic hydricity of cis-Cp(CO)₂-

(PCy₃)MoH. The kinetics were followed by measuring the rate

of appearance of the absorbance from Cp(CO)₂(PCy₃)MoFBF₃

at λ_max) 516 nm (ꢀ ) 6.6 × 10²M^-1cm^-1). The effect of

k_CTk_T

k_obs

)

[Ph₃C⁺]

(7)

+

(

)

k_TC+ k_T[Ph₃C ]

for these experiments with [Ph₃C⁺] . [MH].

Rearrangement of eq 7 gives eq 8.

ionic strength on the rate of hydride transfer was investigated

by measuring the kinetics of reaction of excess [Ph₃C⁺BF₄

]

-

(4.5 mM) with Cp(CO)₂(PCy₃)MoH (0.6 mM). At 15 °C, k_obs

) 65 s^-1. This observed rate constant decreased with increasing

ionic strength (added [NBu₄⁺BF₄^-] ) 5, 10, and 16 mM), but

the changes were small, being only 12% lower (58 s^-1) with

16 mM [NBu₄⁺BF₄^-].

Stopped-flow measurements at various temperatures were

carried out in CH₂Cl₂solution for reaction of Cp(CO)₂(PCy₃)-

MoH (0.2 mM) with excess [Ph₃C⁺BF₄^-] (2-13 mM). The

ionic strength was held constant at 13.5 mM [BF₄^-] by addition

of appropriate amounts of [NBu₄⁺BF₄^-]. Only a small amount

(11%) of the trans isomer is initially present. A steady-state

approximation for trans-Cp(CO)₂(PCy₃)MoH in the mechanism

shown in Scheme 2 leads to the rate expressions shown in eqs

5 and 6 for these experiments that use excess [Ph₃C⁺]. (See the

k_TC

1

k_obs

1

k_CT

)

+

(8)

k_CTk_T[Ph₃C⁺]

Figure 4 shows a plot of 1/k_obsvs 1/[Ph₃C⁺] at temperatures

of -35, -15, and 15 °C. According to eq 8, the slope of this

plot is k_TC/(k_CTk_T) and the intercept is 1/k_CT. At -35 °C the

value of k_CTdetermined from this plot is 8.5 s^-1, compared

with k_CT) 6.2 s^-1determined by NMR. As can be seen in

Figure 4, the y-intercept is very small at 15 °C, so the value of

d[product]

) k_obs[cis-MH]

(5)

(6)

dt

k_CTk_T

k_TC+ k_T[Ph₃C⁺]

where k_obs

)

+ k_C[Ph₃C⁺]

(

)

Appendix for further details). The isomerization rate constants

k_TCand k_CTwere independently determined by NMR as

indicated earlier. By using these values, along with the values

of k_Tobtained as described above, the kinetics data for

experiments reacting excess Ph₃C⁺BF₄^-with Cp(CO)₂(PCy₃)-

MoH at constant ionic strength can be fit to eq 6 to estimate a

value for k_C.²⁸Figure 3 shows a fitting of the data determined

Figure 4. Plot of 1/k_obsvs 1/[Ph₃C⁺BF₄^-] for hydride transfer from

Cp(CO)₂(PCy₃)MoH at -35°, -15°, and 15 °C (13.5 mM [BF₄^-]).

k_CTis not accurately determined at this temperature. The slopes

of the lines provide k_TC/(k_CTk_T) values about twice as large as

those determined more directly from NMR and from the

stopped-flow experiments using excess [Cp(CO)₂(PCy₃)MoH].

One reason for the discrepancy in rate constants may be the

differences in ionic strength used for determination of the

isomerization rate constants vs that used in determining the

hydride transfer rate constants. However, although the agreement

of the values determined from Figure 4 and the independent

NMR and stopped-flow measurements is not exact, it is close

enough to validate the proposed mechanism shown in Scheme

2. These kinetic results therefore provide direct quantitative

confirmation of the much higher reactivity of the trans isomer

than the cis isomer.

NMR Experiments on the Reaction of Cp(CO)₂(PR₃)MoH

with Ph₃C⁺. The reactions of the phosphine-substituted mo-

lybdenum hydrides with Ph₃C⁺BF₄^-were also studied by low-

temperature NMR. These NMR experiments are complementary

to the detailed kinetic data obtained by stopped-flow studies.

Prior studies^25,29of Cp(CO)₂(PR₃)MoX complexes with “four-

legged piano stool” structures established ¹H NMR criteria for

Figure 3. Plot of k_obsvs excess [Ph₃C⁺BF₄^-] for hydride transfer from

Cp(CO)₂(PCy₃)MoH at -55 °C with 13.5 mM [BF₄^-]. The curve shows

the fitting of the data to eq 6.

at -55 °C; from this line-fitting, we determined k_C) 19 M^-1

s^-1. For comparison, k_T) 1.9 × 10⁴M^-1s^-1at -55 °C (at

-55 °C, k_TC) 5.4 s^-1, k_CT) 0.60 s^-1). Thus the rate constant

for hydride transfer from cis-Cp(CO)₂(PCy₃)MoH is about a

thousand times less than that for hydride transfer from trans-

Cp(CO)₂(PCy₃)MoH. At the higher temperatures, where k_C,

(k_CTk_T)/(k_TC+ k_T[Ph₃C⁺]), then eq 6 simplifies to eq 7

1

distinguishing cis and trans isomers. The H NMR shift of the

Cp protons of the trans isomer appears as a doublet (J ≈ 1.9

Hz), typically 0.2-0.3 ppm upfield of the singlet Cp resonance

of isomeric cis complexes. The reaction of Cp(CO)₂(PPh₃)MoH

with Ph₃C⁺BF₄^-was carried out at -78 °C in CD₂Cl₂, and the

NMR tube was immediately inserted into an NMR probe at -80

°C. The predominant product (90%) was trans-Cp(CO)₂(PPh₃)-

MoFBF₃, which exhibited a doublet (J_PF) 19.9 Hz) at δ 66.6

in the ³¹P NMR, in agreement with an earlier report by Beck

(28) The value of k_Cis most accurately determined at relatively high

[Ph₃C⁺]. In the limit of k_T[Ph₃C⁺] . k_TC, eq 6 would simplify to k_obs

)

k_CT+ k_C[Ph₃C⁺]; a plot of k_obsvs [Ph₃C⁺] would give a straight line with

a slope of k_C, as can be seen from Figure 3. Alternatively, at low [Ph₃C⁺],

k_TC. k_T[Ph₃C⁺], and eq 6 would become k_obs) {(k_CTk_T/k_TC) + k_C}[Ph₃C⁺].

Note that since k_CTk_T/k_TC. k_C, k_Cwould be poorly determined.

(29) Kalck, P.; Poilblanc, R. J. Organomet. Chem. 1969, 19, 115-121.

13126 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

and co-workers.³⁰The minor product (10%) exhibited a singlet

in the ³¹P NMR spectrum and is assigned as [trans-Cp(CO)₂-

(PPh₃)Mo(ClCD₂Cl)]⁺BF₄^-, a complex with a weakly coordi-

nating dichloromethane ligand.^21,31The sample was slowly

warmed in the NMR probe; at -30 °C, 2% of cis-Cp(CO)₂-

(PPh₃)MoFBF₃had formed by isomerization of the trans isomer.

Further warming to -8 °C caused more significant changes:

The amount of cis-Cp(CO)₂(PPh₃)MoFBF₃increased to 52%

-80 °C, 95% conversion to cis-Cp(CO)₂(PCy₃)MoFBF₃was

observed; minor peaks presumed to be from trans-Cp(CO)₂-

-

(PCy₃)MoFBF₃(4%) and [Cp(CO)₂(PCy₃)Mo(ClCD₂Cl)]⁺BF₄

(1%) were observed at -80 °C but were no longer seen after

the temperature was raised to -40 °C. The structure of cis-Cp-

(CO)₂(PCy₃)MoFBF₃has been determined by single-crystal

X-ray diffraction and will be reported separately.³³

Hydride transfer from Cp(CO)₂(PCy₃)MoH to Ph₃C⁺OTf^-

at 22 °C in CH₂Cl₂similarly produced cis-Cp(CO)₂(PCy₃)-

MoOTf. In contrast to the results obtained with the PPh₃

complex Cp(CO)₂(PPh₃)MoH, where the trans product pre-

dominated from hydride transfers in CD₃CN, hydride transfer

in CD₃CN from Cp(CO)₂(PCy₃)MoH to Ph₃C⁺BF₄^-, Ph₂-

(MeOC₆H₄)C⁺BF₄^-, or Ph₃C⁺OTf^-consistently gave the ratio

of 85 ((1):15 ((1) for [cis-Cp(CO)₂(PCy₃)Mo(NCCD₃)]⁺:

[trans-Cp(CO)₂(PCy₃)Mo(NCCD₃)]⁺.

and the trans-Cp(CO)₂(PPh₃)MoFBF₃dropped to 45%. The ³¹

P

NMR resonance of each of these isomers was a quintet at -8

-

°C, indicating “spinning” of the BF₄ligand.^30,32After 1 h at

-8 °C, only 5% of trans-Cp(CO)₂(PPh₃)MoFBF₃remained, and

the thermodynamically favored product cis-Cp(CO)₂(PPh₃)-

MoFBF₃had increased to 95%. When the same reaction was

carried out at room temperature and the NMR was recorded at

either room temperature or at low temperature, cis-Cp(CO)₂-

(PPh₃)MoFBF₃was the predominant product observed (∼99:1

cis:trans).

Effect of Counterion on Kinetic Hydricity. The effect of

counterion on the kinetic hydricity was briefly examined for

two metal hydrides. For hydride transfer from Cp(CO)₃MoH,

The reaction of Cp(CO)₂(PPh₃)MoH with Ph₃C⁺OTf^-(OTf

) OSO₂CF₃) at 22 °C gave a 90:10 ratio of trans-Cp(CO)₂-

(PPh₃)MoOTf:cis-Cp(CO)₂(PPh₃)MoOTf. In contrast to the

facile isomerization of trans-Cp(CO)₂(PPh₃)MoFBF₃at low

temperatures, isomerization of the metal triflate trans-Cp(CO)₂-

(PPh₃)MoOTf was very slow, requiring days at room temper-

ature. A similar predominance of the trans product was found

the rate constant found for Ph₃C⁺BF₄in CH₂Cl₂at 25 °C

-

(k ) 3.8 × 10²M^-1s^-1) was quite similar to that found when

using Ph₃C⁺PF₆(k ) 3.5 × 10²M^-1s^-1). A second

-

comparison was made between hydride transfer from Cp-

(CO)₃WH to Ph₃C⁺BF₄compared with Ph₃C⁺BAr′₄[Ar′ )

-

3,5-bis(trifluoromethyl)phenyl]. The rate constant found with

-

Ph₃C⁺BF₄(k ) 76 M^-1s^-1) was somewhat larger than that

from the reaction of Cp(CO)₂(PPh₃)MoH with either Ph₃C⁺BF₄

-

found for Ph₃C⁺BAr′₄(k ) 53 M^-1s^-1). The product from

or Ph₃C⁺OTf^-at room temperature in CD₃CN solventsthe ratio

of [trans-Cp(CO)₂(PPh₃)Mo(NCCD₃)]⁺:[cis-Cp(CO)₂(PPh₃)Mo-

(NCCD₃)]⁺was 96:4 in both cases. Smith and Tilset²⁷studied

the kinetics of isomerization of [trans-Cp(CO)₂(PPh₃)Mo-

(NCCH₃)]⁺to [cis-Cp(CO)₂(PPh₃)Mo(NCCH₃)]⁺and found it

to be very slow (∆H^q) 24.2 ( 0.4 kcal mol^-1; ∆S^q) -3.4 (

1.4 cal K^-1mol^-1; t_1/2) 44 h at 30 °C).

the reaction of Cp(CO)₃WH with Ph₃C⁺BAr′₄was observed

-

by ¹H NMR but was unstable in solution; it is tentatively

assigned as [Cp(CO)₃W(ClCD₂Cl)]⁺BAr′₄

.

-

Kinetic Hydricity of Cp*(CO)₃MoH to Substituted (Sta-

bilized) Trityl Cations. Addition of a methoxy group to the

para position of the phenyl in trityl cation stabilizes the

carbenium ion, making it a weaker hydride acceptor. The effect

of sequential addition of p-MeO groups on Ph₃C⁺was studied

by determining the kinetics of hydride transfer from Cp*-

(CO)₃MoH to trityl cations containing 0, 1, 2, or 3 p-MeO

groups; the results are shown in Table 3. Stabilization of Ph₃C⁺

When the reaction of the PMe₃complex Cp(CO)₂(PMe₃)-

MoH with Ph₃C⁺BF₄was observed by NMR at -80 °C, the

-

dichloromethane complex [trans-Cp(CO)₂(PMe₃)Mo(ClCD₂-

Cl)]⁺BF₄^-made up 26% of the product mixture. About 9% of

the mixture was cis-Cp(CO)₂(PMe₃)MoFBF₃, with trans-Cp-

(CO)₂(PMe₃)MoFBF₃being the predominant product, as in the

PPh₃case. Incremental warming of the sample showed that

conversion of the kinetic products, [trans-Cp(CO)₂(PMe₃)-

Mo(ClCD₂Cl)]⁺BF₄^-and trans-Cp(CO)₂(PMe₃)MoFBF₃, to the

thermodynamic product, cis-Cp(CO)₂(PMe₃)MoFBF₃, was slower

for these PMe₃complexes than for the PPh₃analogues. After

45 min at -10 °C followed by 20 min at 10 °C, the yield of

trans-Cp(CO)₂(PMe₃)MoFBF₃had decreased to 31%, the [trans-

Cp(CO)₂(PMe₃)Mo(ClCD₂Cl)]⁺BF₄^-had decreased to 5%, and

the thermodynamic product cis-Cp(CO)₂(PMe₃)MoFBF₃had

increased to 64%. The thermodynamic product mixture is ∼99:1

cis-Cp(CO)₂(PMe₃)MoFBF₃:trans-Cp(CO)₂(PMe₃)MoFBF₃. The

direct NMR observation of transient [trans-Cp(CO)₂(PR₃)Mo-

Table 3. Rate Constants for Hydride Transfer from Cp*(CO)₃MoH

to Ph_n(p-MeOC₆H₄)_3-nC⁺BF₄(n ) 3, 2, 1, 0) (CH₂Cl₂, 25 °C)

-

k_H

trityl cation

Ph₃C⁺

(M^-1s^-1

)

trityl cation

(M^-1s^-1

)

6.5 × 10³Ph(p-MeOC₆H₄)₂C⁺1.1 × 10¹

Ph₂(p-MeOC₆H₄)C⁺1.4 × 10²(p-MeOC₆H₄)₃C⁺

1.4 × 10⁰

by 3 p-MeO groups causes the rate constant to decrease by a

factor of >4000 from that for unsubstituted Ph₃C⁺. The effect

is largest for addition of the first p-MeO group, which diminishes

the rate constant by more than the subsequent additions.

-

+

A plot of log k_Hvs pK_Ris linear (Figure S10 in the

-

(ClCD₂Cl)]⁺BF₄(R ) Me, Ph) shows that the solvent CD₂-

+

supporting information) with a slope of -0.5. K_Rvalues are a

Cl₂can capture the 16-electron cation [Cp(CO)₂(PR₃)Mo]⁺and

temporarily occupy a coordination site until a stronger ligand

displaces it.

traditional measurement of stability of carbenium ions³⁴and are

+

based on measurements in aqueous sulfuric acid; the K_Rvalues

are defined according to eq 9. The excellent linearity of the

A strikingly different result was obtained for the reaction of

the PCy₃complex Cp(CO)₂(PCy₃)MoH with Ph₃C⁺BF₄^-at -78

°C in CD₂Cl₂. When this reaction was monitored by NMR at

K_R₊

R⁺+ H₂O y z ROH + H⁺

(9)

-

+

plot of of log k_Hvs pK_Ris surprising in view of the very

(30) Su¨nkel, K.; Urban, G.; Beck, W. J. Organomet. Chem. 1983, 252,

187-194.

different solvents used in obtaining the kinetic data (CH₂Cl₂),

(31) Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89-

115.

(33) Cheng, T.-Y.; Szalda, D. J.; Bullock, R. M., manuscript in

preparation.

(34) Carbonium Ions; Olah, G. A.; Schleyer, P. v. R., Eds.; Wiley and

Sons: New York, 1968; Vol. I.

(32) (a) Honeychuck, R. V.; Hersh, W. H. J. Am. Chem. Soc. 1989, 111,

6056-6070. (b) Honeychuck, R. V.; Hersh, W. H. Inorg. Chem. 1989, 28,

2869-2886.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13127

compared with the thermodynamic determinations of carbenium

ion stability in aqueous acidic solution.

Effect of the Ligands on Kinetic Hydricity. Systematic

changes in the ligands allowed the evaluation of the effect of

steric and electronic properties of different ligands on the same

metal. A series of tungsten compounds containing Cp and

substituted Cp ligands showed a dramatic change in hydricity

as the electronic properties of the ligands were changed. The

range of kinetic hydricity spans over 3 orders of magnitude for

these compounds, with the rate constants decreasing in the order

(indenyl)(CO)₃WH ≈ Cp*(CO)₃WH > (C₅H₄Me)(CO)₃WH >

Cp(CO)₃WH > (C₅H₄CO₂Me)(CO)₃WH. There is clearly a

substantial electronic effect caused by changing the electron

density at the substituted Cp, which in turn affects the hydricity

of the complex. Compared with the Cp ligand, the Cp* ligand

is much larger sterically and is a better electron donor. The larger

rate constants for Cp*M(CO)₃H than for CpM(CO)₃H

(k_Cp*/k_Cp) 25 for W; k_Cp*/k_Cp) 17 for Mo) indicate that

electronic effects clearly predominate over steric effects. Even

addition of one methyl group to the Cp ligand increases the

hydricity, as seen by the hydricity of (C₅H₄Me)(CO)₃WH being

3 times more than that of Cp(CO)₃WH. An even larger effect

from the substitution of a single substituent on the Cp ligand

was found for the compound containing an electron-withdrawing

group on the Cp ligand, (C₅H₄CO₂Me)(CO)₃WH being ∼100

times slower at hydride transfer than Cp(CO)₃WH.

Dimethylamino groups stabilize carbenium ions even more

+

effectively than methoxy groups. The pK_Rvalue for (p-Me₂-

NC₆H₄)₃C⁺is 9.36,³⁴compared with 0.82 for (p-MeOC₆H₄)₃C⁺

and -6.63 for Ph₃C⁺. The reaction of Cp(CO)₂(PMe₃)MoH with

Ph(p-Me₂NC₆H₄)₂C⁺BF₄^-at room temperature was very slow,

giving ∼67% Ph(p-Me₂NC₆H₄)₂CH after 1 h. Several unidenti-

fied organometallic products were formed, but no significant

amount of Cp(CO)₂(PMe₃)MoFBF₃was observed by NMR. The

-

reaction of (p-Me₂NC₆H₄)₃C⁺BF₄with Cp(CO)₂(PMe₃)MoH

was even slower, leading to only ∼6% (p-Me₂NC₆H₄)₃CH after

2 days.

Discussion

Effect of the Metal on Kinetic Hydricity. The kinetic data

reported here allow the direct comparison of kinetic hydricity

of different metals with identical ligands. In all cases examined

here, third-row metal hydrides exhibit faster rates of hydride

transfer than their first-row analogues. For example, the third-

row tungsten hydride Cp*(CO)₃WH is 33 times faster at hydride

transfer than its first-row analogue Cp*(CO)₃CrH. A similar

ratio is found for Re vs Mn, with (CO)₅ReH being 40 times

faster than (CO)₅MnH. The relative reactivities are similar for

the PPh₃-substituted examples, with cis-(CO)₄(PPh₃)ReH being

52 times faster than cis-(CO)₄(PPh₃)MnH.

In both cases where our data allow direct comparisons, the

second-row metal hydrides are even faster at hydride transfer

than third-row analogues, though the differences in rate constants

are less than the differences between the first and third rows.

Thus Cp*(CO)₃MoH is ∼3 times faster than the tungsten

compound Cp*(CO)₃WH; a similar reactivity ratio was found

for the analogous Cp compounds, with Cp(CO)₃MoH being 5

times faster than Cp(CO)₃WH.

Comparisons of reactivity of indenyl (η⁵-C₉H₇) complexes

with related Cp or Cp* complexes have been of much interest,

owing in large part to the propensity of indenyl complexes to

undergo η⁵f η³“ring slippage” reactions³⁷that dramatically

enhance ligand substitution rates. On the basis of electrochemical

comparisons of substituted ruthenocenes containing Cp/Cp*/

indenyl ligands, Gassman and co-workers concluded that indenyl

was similar to Cp* in electron donation to the metal.³⁸The

electron-donating ability of the indenyl ligand has been ranked

as greater than Cp and less than Cp* on the basis of gas-phase

measurements using ion cyclotron resonance³⁹and photoelectron

spectra.⁴⁰Our kinetic data indicate comparable hydridic reactiv-

ity of (indenyl)(CO)₃WH and Cp*(CO)₃WH, both of these being

much faster hydride donors than the unsubstituted Cp complex

Cp(CO)₃WH.

These comparisons of the effect of the metal on our neutral

metal carbonyl hydrides can be compared with earlier observa-

tions made by Darensbourg and Ash on hydride transfer

reactions of anionic metal carbonyl hydrides.⁵Reduction of

n-butyl bromide with anionic metal hydrides gives the hydro-

carbon product and converts the organometallic product to an

anionic metal bromide (eq 10).⁶The kinetics of these reactions

Substitution of one CO ligand by a phosphine can dramati-

cally affect the kinetic hydricity of the metal hydride. The PPh₃-

substituted Mo hydride trans-Cp(CO)₂(PPh₃)MoH is ∼1500

times faster at hydride transfer than Cp(CO)₃MoH. The elec-

tronic effect of the phosphine is to make the metal center more

electron-rich, and this effect is manifested in the greatly

enhanced kinetic hydricity of the phosphine-substituted metal

hydride. The electronic effect clearly predominates over the

steric effect of the phosphine. An even larger enhancement is

found by substituting 1 CO for PMe₃, trans-Cp(CO)₂(PMe₃)-

MoH being 10 000 times as reactive as Cp(CO)₃MoH. The

electronic effect of the PMe₃makes its hydride complex even

more hydridic than the PPh₃-substituted analogue but with less

steric interference than the PPh₃-substituted compound. In metal

hydrides with very bulky phosphines, steric effects can have

MH^-+ n-BuBr f MBr^-+ n-BuH

(10)

were found to be second-order, with k ≈ 10^-2-10^-3M^-1s^-1

at 26 °C in tetrahydrofuran (THF). The rate constant for

(CO)₅WH^-was 1.8 times larger than that for (CO)₅CrH^-. The

P(OMe)₃-substituted W and Cr hydrides had a similar relative

ratio, with (CO)₄[P(OMe)₃]WH^-being ∼1.7 times faster than

(CO)₄[P(OMe)₃]CrH^-.

The main-group hydride HSiEt₃is frequently used as a

hydride donor in ionic hydrogenations of organic substrates.³⁵

2

Our measurement of k_H) 1.5 × 10 M^-1s^-1for HSiEt₃places

-

this silane hydride donor as slower than most of the transition

metal hydride donors (Table 1). Mayr and co-workers reported

detailed kinetic studies of hydride transfers to diarylcarbenium

ions from a series of silyl hydrides and evaluated the effect of

substituent effects on the hydridic reactivity of several silyl

hydrides.³⁶

some influence on the kinetic hydricity. PCy₃(cone angle⁴¹

θ

) 170°) is much more sterically demanding than PMe₃(θ )

118°), and trans-Cp(CO)₂(PCy₃)MoH is ∼10 times slower than

trans-Cp(CO)₂(PMe₃)MoH at hydride transfer.

(37) O′Connor, J. M.; Casey, C. P. Chem. ReV. 1987, 87, 307-318.

(38) Gassman, P. G.; Sowa, J. R. Jr.; Hill, M. G.; Mann, K. R.

Organometallics 1995, 14, 4879-4885.

(35) (a) Kursanov, D. N.; Parnes, Z. N.; Loim, N. M. Synthesis 1974,

633-651. (b) Kursanov, D. N.; Parnes, Z. N.; Kalinkin, M. I.; Loim, N.

M. Ionic Hydrogenation and Related Reactions; Harwood Academic

Publishers: New York, 1985.

(36) Mayr, H.; Basso, N.; Hagen, G. J. Am. Chem. Soc. 1992, 114, 3060-

3066.

(39) Ryan, M. F.; Siedle, A. R.; Burk, M. J.; Richardson, D. E.

Organometallics 1992, 11, 4231-4237.

(40) Crossley, N. S.; Green, J. C.; Nagy, A.; Stringer, G. J. Chem. Soc.,

Dalton Trans. 1989, 2139-2147.

(41) Tolman, C. A. Chem. ReV. 1977, 77, 313-348.

13128 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

For the PCy₃-substituted Mo complex Cp(CO)₂(PCy₃)MoH,

we established that the kinetic hydricity of the trans isomer is

much higher than that of the cis isomer. Our measurements at

-55 °C (the only temperature at which we were able to

determine the kinetic hydricity of the cis isomer) showed that

trans-Cp(CO)₂(PCy₃)MoH is ∼1000 times faster at hydride

transfer than is cis-Cp(CO)₂(PCy₃)MoH. The electronic effect

of the PCy₃phosphine in activating trans-Cp(CO)₂(PCy₃)MoH

toward hydride transfer is essentially canceled out by the steric

effect in the cis isomer of the same compound, such that the

rate constant of cis-Cp(CO)₂(PCy₃)MoH for hydride transfer is

similar to that of the unsubstituted complex Cp(CO)₃MoH. We

have not accounted for different intrinsic electronic effects that

the phosphine may have on the kinetic hydricity of cis vs trans

isomers in these “piano stool” structures, so this factor might

influence the relative reactivities as well.

cis-[Cp(CO)₂(PR₃)M(NCCH₃)]⁺required many hours. We found

a similar predominance of trans-Cp(CO)₂(PPh₃)MoOTf from

the reaction of Cp(CO)₂(PPh₃)MoH with Ph₃C⁺OTf^-. In these

cases, the 16-electron [Cp(CO)₂(PR₃)M]⁺produced by hydride

transfer is captured by a strongly binding ligand, CH₃CN or

OTf^-, and subsequent trans f cis isomerization is slow. In

contrast to these cases where trans products were observed as

kinetic products, hydride abstractions from Cp(CO)₂(PCy₃)MoH

consistently gave a predominance of cis products. This was the

case not only in trapping of the 16-electron cation [Cp(CO)₂-

(PCy₃)Mo]⁺by the strongly binding ligands CH₃CN and OTf^-

but also in hydride transfer from Cp(CO)₂(PCy₃)MoH to

Ph₃C⁺BF₄^-, which gave predominantly cis-Cp(CO)₂(PCy₃)-

MoFBF₃. These results clearly indicate that use of stereochem-

istry of the initially observed product is not an infallible indicator

of the site from which the hydride was donated, since we

observe cis products from hydride transfer from Cp(CO)₂(PCy₃)-

MoH, despite having shown by the kinetics that trans-Cp(CO)₂-

(PCy₃)MoH is far more reactive than cis-Cp(CO)₂(PCy₃)MoH.

Our results also provide an evaluation of the effect of

phosphine substitution in six-coordinate Mn and Re complexes.

The PPh₃-substituted Mn complex cis-(CO)₄(PPh₃)MnH is more

hydridic than (CO)₅MnH by ∼5-fold; a similar effect is observed

for the Re analogues. The effect of phosphine substitution on

the hydricity of these Mn and Re complexes may be compared

to the effect of phosphite substitution on anionic metal hydrides

(CO)₄[P(OMe)₃]MH^-studied by Darensbourg (eq 10), since

both of classes of metal hydrides have approximately octahedral

geometry, the phosphorus ligand positioned cis to the hydride.

The phosphite-substituted Cr hydride (CO)₄[P(OMe)₃]CrH^-is

∼17 times faster at reduction of n-BuBr (eq 10) than (CO)₅CrH^-.⁶

Reactivity of Free vs Ion-Paired [Ph₃C⁺]. Several studies

have measured dissociation constants (K_D) for formation of free

ions from ion pairs of trityl cation.⁴²The dissociation constant

The observation of cis products from Cp(CO)₂(PCy₃)MoH

could be due to fast trans f cis isomerization of “[Cp(CO)₂-

(PCy₃)Mo]⁺” prior to capture by either CH₃CN, OTf^-, or BF₄

.

-

Alternatively, these results are also consistent with initial capture

of “[Cp(CO)₂(PCy₃)Mo]⁺” to give a trans product. Dissociation

of CH₃CN, OTf^-, or BF₄^-from the trans kinetic product could

be followed by trans f cis isomerization and reassociation of

the ligand. A higher rate of trans f cis isomerization of the

PCy₃-substituted complex is corroborated by our observation

of a higher rate of conversion of kinetic products [trans-Cp-

(CO)₂LMo(ClCD₂Cl)]⁺BF₄and trans-Cp(CO)₂LMoFBF₃to

-

the thermodynamic product cis-Cp(CO)₂LMoFBF₃for L ) PPh₃

than for L ) PMe₃. The implication is that larger steric demands

of phosphines accelerate the isomerization.

reported for [Ph₃C⁺SbCl₆^-] in CH₂Cl₂at 25 °C is K_D) 1.6 ×

10^-4M (eq 11).^42aTypical initial concentrations of [Ph₃C⁺BF₄

]

-

Comparison of Acidity vs Hydricity of Metal Carbonyl

Hydrides. An understanding of the factors influencing the

kinetic and thermodynamic acidity of metal hydrides is available,

notably from the work of Norton’s group.¹⁰Our results on the

kinetic hydricity of many of the same metal hydrides allow

certain comparisons to be made. The notion that relative

propensities of proton transfer and hydride transfer modes of

M-H bond cleavage might be roughly the opposite of each

other is tempered by several factors.⁴⁴Deprotonation of a metal

hydride gives a metal anion that is generally a stable 18-electron

species. In contrast, hydride transfer from MH gives a formally

16-electron “M⁺” that differs in electron count from the metal

hydride and that almost invariably requires coordination with

another ligand to form a stable species. The kinetics of proton

transfer from a series of metal hydrides to aniline in CH₃CN at

25 °C have been reported.⁴⁵Figure 5 shows a plot comparing

rate constants for proton-transfer reactions with those for our

K_D

\

[Ph₃C⁺SbCl₆] y

z [Ph₃C⁺] + [SbCl₆]

(11)

-

used in our kinetics experiments were 0.05 mM. If we assume

that K_Dis similar for Ph₃C⁺SbCl₆and Ph₃C⁺BF₄^-, our

-

conditions would initially involve ∼80% free ions and 20% ion

pairs. The relative amount of free ions increases as the reaction

proceeds, because of consumption of the Ph₃C⁺BF₄^-. Since our

kinetics exhibit very clean first-order rates for disappearance

of [Ph₃C⁺BF₄^-], there is no evidence for any difference in

reactivity of free and ion-paired forms of Ph₃C⁺BF₄^-. A

negligible dependence on counterion (BF₄^-, PF₆^-, AsF₆

,

-

SbF₆^-, SbCl₆^-) or extent of ion pairing was previously observed

in hydride transfers to Ph₃C⁺from HSiEt₃.²²Mayr and co-

workers noted the absence of ion-pairing effects in the kinetics

of addition of alkenes to diarylcarbenium ions,⁴³so comparable

reactivity of free and ion-paired carbenium ions appears to have

some generality in CH₂Cl₂solution.

Capture of the 16-Electron “M⁺” after Hydride Transfer.

As discussed earlier, Tilset and co-workers found highly

selective formation of the kinetic product trans-[Cp(CO)₂-

(PR₃)M(NCCH₃)]⁺after hydride transfer to substituted trityl

cations from either Cp(CO)₂(PMe₃)WH¹⁹or Cp(CO)₂(PPh₃)-

MoH.²⁷Isomerization to the thermodynamically favored product

(42) (a) Gogolczyk, W.; Slomkowski, S.; Penczek, S. J. Chem. Soc.,

Perkin Trans. 2 1977, 1729-1731. (b) Kalfoglou, N.; Szwarc, M. J. Phys.

Chem. 1968, 72, 2233-2234. (c) Bowyer, P. M.; Ledwith, A.; Sherrington,

D. C. J. Chem. Soc. (B) 1971, 1511-1514. (d) Lee, W. Y.; Treloar, F. E.

J. Phys. Chem. 1969, 73, 2458-2459. (e) Schneider, R.; Mayr, H.; Plesch,

P. H. Ber. Gunsenges. Phys. Chem. 1987, 91, 1369-1374.

(43) (a) Mayr, H.; Schneider, R.; Schade, C.; Bartl, J.; Bederke, R. J.

Am. Chem. Soc. 1990, 112, 4446-4454. (b) Mayr, H. Angew. Chem., Int.

Ed. Engl. 1990, 29, 1371-1384.

-

+

Figure 5. Plot of log k_Hvs log k_Hcomparing kinetic hydricity with

kinetic acidity.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13129

hydride transfer reactions. Some ligand effects on acidity vs

hydricity support the idea that more electron-donating ligands

enhance hydricity and diminish kinetic acidity, as seen by Cp*-

(CO)₃MoH being more hydridic and less acidic than Cp-

(CO)₃MoH. A similar argument holds for (CO)₄(PPh₃)MnH and

(CO)₅MnH. The lack of a more general correlation among the

different metals and ligand sets shown in Figure 5 suggests,

however, that too many factors influence the acidity and the

hydricity for any simple relationship to be established confi-

dently, at least on the basis of presently available data.

Comparison of Kinetic Hydricity with Bond Strengths.

The M-H bond dissociation energies (BDE’s) of many of the

metal hydrides studied in this work have been reported, mostly

through the use of thermochemical cycles utilizing electro-

chemical data and pK_adata of the metal hydrides.^19,46Figure 6

Comparison of Hydride Transfer with Hydrogen Atom

Transfer. Norton and co-workers reported a detailed study of

the kinetics of hydrogen atom transfer from a series of metal

hydrides to the stable radical tris(p-tert-butylphenyl)methyl (eq

12).¹²Steric considerations are expected to be roughly similar

(12)

for their substituted trityl radical, compared with the Ph₃C⁺used

in our hydride transfers. Their reactions were carried out in

toluene, whereas our rate constants were determined in CH₂-

Cl₂. Despite these differences, some comparisons of relative rate

constants for these reactions may be useful for the four metal

hydrides examined in both studies. Second-order rate constants

(k) reported at 25 °C for eq 12 were 741 M^-1s^-1for (CO)₅-

MnH, 514 M^-1s^-1for Cp(CO)₃MoH, 91 M^-1s^-1for Cp(CO)₃-

WH, and 13.9 M^-1s^-1for Cp*(CO)₃MoH. Of these four metal

hydrides, (CO)₅MnH has the highest rate constant for hydrogen

atom transfer, but it is the slowest at hydride transfer. The Cp

complex Cp(CO)₃MoH is 37 times faster at hydrogen atom

transfer than is its Cp* analogue Cp*(CO)₃MoH, but the Cp*

complex has a higher rate constant for hydride transfer.

Apparently the enhanced hydricity is due to the electronic effect

of the Cp* ligand, compared with that of the Cp ligand. For

both hydrogen atom and hydride transfer reactions, the Mo

hydride, Cp(CO)₃MoH, reacts faster than the W analogue, Cp-

(CO)₃WH.

Mechanism of Hydride Transfer: Single-Step Hydride

Transfer vs Initial Electron Transfer. Darensbourg and Ash

noted that hydricity “is difficult to define experimentally because

of the multiple pathways available for an overall H^-transfer.”⁵

We have been referring to hydricity in this paper with the

implication that these reactions proceed by hydride transfer from

the metal to carbon in a single step. An alternative mechanistic

pathway would be an initial electron transfer, followed by

hydrogen atom transfer. We now consider the viability of such

a mechanism as shown in Scheme 3. The first step is simply

-

Figure 6. Plot of ln k_Hvs homolytic bond dissociation energies of

M-H.

-

shows a plot of ln k_Hvs the homolytic bond dissociation

energy.^47,48For example, the reported bond dissociation energies

of (CO)₅MnH, (CO)₄(PPh₃)MnH, Cp(CO)₃MoH, and Cp*-

(CO)₃MoH differ by only ∼1 kcal mol^-1, yet their kinetic

hydricity range spans a factor of >100. The kinetic hydricity

of Cp*(CO)₃CrH, (CO)₅MnH, and Cp(CO)₃WH span a range

of only 1.5, whereas their reported BDEs differ by 10 kcal

mol^-1. Clearly, there is no overall correlation of the kinetic

hydricity, which involves a heterolytic M-H bond cleavage,

with the thermodynamics of the homolytic BDEs.

Scheme 3

Although the value of thermochemical data in interpreting

kinetic trends is immense, most of the data are confined to

homolytic bond dissociation energies. Berke and co-workers

have reported deuterium quadrupole coupling constants for a

series of metal deuterides, from which they calculated the

“ionicity” of the M-D bonds.⁴⁹Some data are available for

hydride affinities of carbenium ions in solution: ∆G_hydride

)

diffusion of the metal hydride and the trityl cation together.

We do not expect a particularly strong affinity of the two

reagents, so K_assoc< 10 M^-1is estimated. The electron transfer

and hydrogen atom transfer steps are the key steps and will be

considered in detail below. The final steps are dissociation of

Ph₃CH from M⁺and the product-forming capture of M⁺by

-96 kcal/mol for Ph₃C⁺in dimethyl sulfoxide.⁵⁰Thermo-

dynamic data on heterolytic bond energies of metal hydrides

would be very useful. Bruno and co-workers recently measured

equilibrium constants for hydride transfers from metal hydrides

to a series of substituted trityl cations in MeCN solution.⁵¹

-

BF₄to produce M-FBF₃. A steady-state approximation for

(44) Labinger, J. A. In Transition Metal Hydrides; Dedieu, A., Ed.;

VCH: New York, 1992; Chapter 10, pp 361-379.

(45) Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1987,

109, 3945-3953.

(46) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711-6717,

as modified in J. Am. Chem. Soc. 1990, 112, 2843.

(47) Parker, V. D.; Handoo, K. L.; Roness, F.; Tilset, M. J. Am. Chem.

Soc. 1991, 113, 7493-7498.

(48) Tilset, M. J. Am. Chem. Soc. 1992, 114, 2740-2741.

(49) Nietlispach, D.; Bakhmutov, V. I.; Berke, H. J. Am. Chem. Soc.

1993, 115, 9191-9195.

(50) Cheng, J.-P.; Handoo, K. L.; Parker, V. D. J. Am. Chem. Soc. 1993,

115, 2655-2660.

{Ph₃C‚|MH^+•} gives eq 13 as the rate constant for hydride

transfer.

We now consider two limits based on relative magnitudes of

k_-etand k_H‚. If k_-et. k_H‚, then k_obs) (K_assoc)(K_et)(k_H‚), with

(51) Sarker, N.; Bruno, J. W., submitted for publication. We thank

Professor Bruno (Wesleyan University) for a preprint of this paper, and for

permission to mention these results. For thermodynamic studies on hydride

affinity of a series of substituted trityl cations as hydride acceptors, see

Zhang, X.-M.; Bruno, J. W.; Enyinnaya, E. J. Org. Chem. 1998, 63, 4671-

4678.

13130 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

minor and would not change our conclusions. Note that in our

reaction, the work terms are expected to be negligible, since

the reaction involves one neutral and one cationic species.⁵⁴

k_et

k_obs) K_assoc

k_H‚

(13)

(

)

k_-et+ k_H‚

While the electron transfer mechanism is disfavored for our

system involving hydride transfer from metal to carbon, we note

that it is quite viable for many other related reactions. Carbon-

to-carbon hydride transfers from dihydronicotinamides are a very

actively studied class of hydride transfer reactions for which

mechanistic questions of electron transfer vs single-step hydride

transfer have been considered in detail.³Kochi and co-workers

recently reported kinetics of hydrogen atom transfers from

methylbenzenes to photoactivated quinones. They concluded that

substantial deuterium kinetic isotope effects are consistent with

a mechanism initiated by electron transfer and followed by

proton transfer as the product-forming step.⁵⁵

Rigorous exclusion of the electron transfer mechanism is less

secure for the phosphine-substituted metal hydrides. Phosphine-

substitution makes the metal hydride more easily oxidized:

oxidation of Cp(CO)₂(PPh₃)MoH was reported at E ≈ +0.26

V (vs Cp₂Fe/Cp₂Fe⁺reference).²⁷Thus oxidation of this metal

hydride by Ph₃C⁺would be unfavorable by only ∼0.3 V (∆G°

≈ 7 kcal mol^-1, K_et≈ 10^-6). However, the observed rate

constants for hydride transfer are larger for the phosphine-

substituted hydrides. The kinetic isotope effect for hydride

transfer from Cp(CO)₂(PCy₃)MoH is k_MoH/k_MoD) 1.7 (see

Figure S8 in the supporting information),²³which is essentially

the same as that found for Cp(CO)₃MoH. For the phosphine-

substituted metal hydride to undergo hydride transfer by an

electron transfer pathway would therefore require that the

mechanism would change but the isotope effect would remain

the same, and this seems untenable. Although the electron

transfer mechanism is not rigorously excluded on the basis of

the kinetic and electrochemical/thermodynamic data for the

phosphine-substituted metal hydrides, we suggest that all of the

hydride transfer reactions in this paper proceed by a single-

step hydride transfer and not by a mechanism initiated by

electron transfer. In recent related studies, Smith, Norton, and

Tilset⁵⁶concluded that hydride transfer from Cp(CO)₂(PPh₃)-

MoH to protonated acetone also occurs by a single-step rather

than an electron transfer mechanism.

the hydrogen atom transfer step being rate-limiting. The

equilibrium constant for electron transfer from the metal hydride

to Ph₃C⁺can be estimated from electrochemical measurements.

Reduction of Ph₃C⁺PF₆in CH₃CN was studied by cyclic

-

voltammetry and was found to be partially reversible. An

estimate of E° ≈ +0.27 V (vs a saturated calomel electrode)

was given;⁵²conversion to a Cp₂Fe/Cp₂Fe⁺reference gives E°

≈ -0.08 V. Oxidation of Cp(CO)₃MoH was reported by Tilset

and co-workers to be quasi-reversible, with E ) 0.800 V (vs

Cp₂Fe/Cp₂Fe⁺) reported for the peak potential for oxidation of

this hydride.¹⁹From these electrochemical data, the oxidation

of Cp(CO)₃MoH by Ph₃C⁺is estimated to be thermodynamically

unfavorable by ∼0.9 V (∆G° ≈ 21 kcal mol^-1), leading to an

estimate of K_et≈ 10^-16(see discussion below). This very

unfavorable electron transfer could be followed by hydrogen

atom transfer from the oxidized metal hydride to the trityl

radical. While oxidation of metal hydrides often results in proton

transfers from the very acidic 17-electron MH^+•species, a

substantial activation of M-H bonds toward homolytic cleavage

can also result from one-electron oxidation.^19,48,53A very fast

hydrogen atom transfer could conceivably occur on the time

scale of a molecular vibration, so we set an upper limit of k_H‚

< 10¹³s^-1(note that k_H‚, k_-etalso). Based on these estimates,

an upper limit estimated for the rate constant of this mechanism

is given in eq 14:

(K_assoc) (K_et) (k_H‚) ) (10 M^-1) (10^-16) (10¹³s^-1) )

10^-2M^-1s^-1(14)

The resulting estimated maximum rate constant is 10^-2M^-1

s^-1, which is 4 orders of magnitude lower than the experimen-

tally measured rate constant of 3.8 × 10²M^-1s^-1for Cp(CO)₃-

MoH. The possibility of preequilibrium electron transfer,

followed by rate-limiting hydrogen atom transfer, is therefore

deemed unlikely.

We now consider the other limiting case of eq 13, rate-

limiting electron transfer. If k_-et, k_H‚, then eq 13 simplifies to

k_obs) (K_assoc)(k_et). Using k_-ete 10¹³s^-1and K_et≈ 10^-16, the

estimated upper limit for the rate constant for electron transfer

is k_et< 10^-3s^-1. The estimated maximum overall rate constant

is then k_obse (K_assoc)(k_et) ) 10^-2M^-1s^-1, the same upper limit

that was estimated for rate-limiting hydrogen atom transfer. We

conclude that hydride transfer from Cp(CO)₃MoH to Ph₃C⁺

proceeds by a single-step hydride transfer, rather than the

mechanism shown in Scheme 3. A further argument against the

possibility of rate-determining electron transfer is that isotope

effects as large as found for these systems [k_MoH/k_MoDof 1.8

for Cp(CO)₃MoH]²³are quite unlikely in electron transfer

reactions, where the M-H (M-D) bond is not cleaved.

There are potential limitations to the use of these electro-

chemical measurements, since the oxidations of the metal

hydrides are usually irreversible. In addition, the use of

electrochemical data to obtain thermodynamic quantities neglects

the work terms, since the thermodynamic quantities derived from

them are for separated freely diffusing species, not caged

intermediates. We contend that such corrections are relatively

Hembre and co-workers reported several interesting observa-

tions on Cp*(dppf)RuH [dppf ) 1,1′-(bis(diphenylphosphino)-

ferrocene], an unusual metal hydride that exhibits two electro-

chemically reversible oxidations.⁵⁷Hydride transfer from

Cp*(dppf)RuH to N-methylacridinium was found to occur by

a single-step hydride transfer,²but hydride transfer to Ph₃C⁺

was by an initial electron transfer followed by hydrogen atom

transfer. Oxidation of metal hydrides becomes easier as the metal

becomes more electron-rich. Whereas we argued above that our

mono-phosphine-substituted hydrides such as Cp(CO)₂(PPh₃)-

MoH undergo single-step hydride transfer, apparently metal

hydrides with two phosphines will more generally undergo

single-electron transfer when reacted with Ph₃C⁺. In further

support of this trend, we recently obtained evidence that reaction

of Ph₃C⁺with Cp(CO)(dppe)MoH (dppe ) Ph₂PCH₂CH₂PPh₂)

(54) For examples where work terms were calculated, see: (a) Sutin,

N.; Brunschwig, B. S. In Mechanistic Aspects of Inorganic Reactions;

Rorabacher, D. B., Endicott, J. F., Eds.; ACS Symposium Series Vol. 198,

American Chemical Society: Washington, DC, 1982; pp 105-133. (b)

Skoog, S. J.; Gladfelter, W. L. J. Am. Chem. Soc. 1997, 119, 11049-11060.

(55) Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc.

1998, 120, 2826-2830.

(56) Smith, K.-T.; Norton, J. R.; Tilset, M. Organometallics 1996, 15,

4515-4520.

(57) Hembre, R. T.; McQueen, J. S.; Day, V. W. J. Am. Chem. Soc.

1996, 118, 798-803.

(52) Asaro, M. F.; Bodner, G. S.; Gladysz, J. A.; Cooper, S. R.; Cooper,

N. J. Organometallics 1985, 4, 1020-1024. For earlier studies of reduction

of a series of substituted trityl cations by polarography, see Volz, H.; Lotsch,

W. Tetrahedron Lett. 1969, 2275-2278.

(53) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077-5083.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13131

in CH₃CN proceeds by an initial electron transfer mechanism,

rather than single-step hydride transfer.⁵⁸The reaction of Ph₃C⁺

with certain metal hydrides containing >2 phosphorus ligands

is known to result in oxidation to give stable MH^+•species,

rather than hydride transfer, as shown for HCo[P(OPh)₃]₄⁵⁹and

HFeCl(dppe)₂.⁶⁰Hydride transfer from some metal alkyl

complexes [e.g., Cp(CO)(NO)ReR⁶¹and Cp₂W(CH₃)₂⁶²] to

Ph₃C⁺also can occur by initial electron transfer.

(15)

We consider one additional mechanistic issue of the hydride

transfer. Because most of our kinetics were carried out with

Ph₃C⁺BF₄^-, there may be a possible additional role of the

without a complete hydride transfer taking place. An example

is shown in eq 15, in which the nucleophilic displacement of

an η¹-aldehyde ligand occurs, leading to a bridging hydride

complex.⁶⁷Nucleophilic displacement of a weakly bound ligand

by a metal hydride is a synthetic procedure that has been

successfully used in many cases for preparing M(µ-H)M′

complexes.⁶⁸A quantitative assessment of the nucleophilicity

of several metal hydrides was carried out by Norton and co-

workers, who reacted an acyl complex of Re (solvated by CD₃-

CN) with a series of metal hydrides.⁶⁹The proposed intermediate

(or transition state) has a three-center two-electron bond like

that found in other M(µ-H)M′ complexes mentioned above; the

final products are the aldehyde and a bimetallic complex

(Scheme 4). Kinetic and mechanistic data indicated that the

-

counterion. Dissociation of BF₄would give F^-and BF₃, and

the BF₃could potentially interact with the metal hydride to

increase its hydricity. However, the kinetic competence of this

possible catalysis of hydride transfer by BF₃is ruled out by the

modest effect observed in kinetics experiments where BF₄^-was

added to hydride transfers of Cp(CO)₂(PCy₃)MoH; in these

experiments, the rate constants decreased slightly with higher

-

amounts of BF₄

.

Comparison of Hydricity of Cp(NO)₂WH with Cp-

(CO)₃WH. The kinetic hydricity of the tungsten nitrosyl hydride

Cp(NO)₂WH was determined to provide a quantitative com-

parison with that of the isoelectronic carbonyl hydride Cp-

(CO)₃WH. Legzdins and Martin first prepared Cp(NO)₂WH and

noted its hydridic reactivity.⁶³The reactivity of metal nitrosyl

hydrides was reviewed by Berke and Burger,⁶⁴who attributed

enhanced hydricity to metal nitrosyl hydrides compared to

similar metal carbonyl hydrides. Bursten and Gatter explained

the hydridic vs acidic differences in reactivity on the basis of

results of molecular orbital studies of the Cr analogues, Cp-

(NO)₂CrH and Cp(CO)₃CrH.⁶⁵Our results show that the kinetic

hydricity of Cp(NO)₂WH exceeds that of Cp(CO)₃WH by ∼250-

fold. We believe that a substantial reason for the difference in

hydricity is the difference in formal oxidation state of the metal.

Counting the nitrosyl ligands as NO⁺results in a formal

oxidation state of W(0) for Cp(NO)₂WH, compared with a

formal oxidation state of W(II) for Cp(CO)₃WH.

Scheme 4

mechanism involved dissociation of the CD₃CN ligand prior to

reaction of the metal hydride; accordingly, the relative rate

constants measured in their study reflected nucleophilicity of

the metal hydrides toward the coordinatively unsaturated Re

acyl complex. The relative rate constants exhibited interesting

trends, and the authors noted that the order of relative nucleo-

philicity was “largely the reVerse of the order of increasing

reactivity of proton transfer onto aniline.”⁶⁹The range of rate

constants measured in Scheme 4 is only ∼2 orders of magnitude,

but the trends in nucleophilic reactivity are strikingly different

from the trends in kinetic hydricity toward Ph₃C⁺. The rhenium

hydride (CO)₅ReH is only 1.4 times as nucleophilic (Scheme

4) as (CO)₅MnH but shows a 40-fold greater rate constant of

kinetic hydricity toward Ph₃C⁺. An even more dramatic

difference is found when comparing the effect of phosphine

substitution: The phosphine-substituted tungsten hydride Cp-

(CO)₂(PMe₃)WH is less nucleophilic than Cp(CO)₃WH (by a

factor of ∼2) in Scheme 4, compared with an acceleration of

10⁴in hydride transfer rate constant for the molybdenum hydride

trans-Cp(CO)₂(PMe₃)MoH vs Cp(CO)₃MoH.

Hydricity vs Nucleophilicity of Metal Hydrides. The

reactions reported in this paper clearly involve hydride transfer,

since the M-H bond is cleaved as a hydride. Many discussions

of reactivity patterns of metal hydrides refer to nucleophilicity

of the metal hydride, with the terms “hydricity” and “nucleo-

philicity” apparently being used interchangeably. We consider

the kinetics reported in this paper to be a true measure of

hydricity, since there is no nucleophilic displacement of any

-

“leaving group” (except for the BF₄counterion that is ion-

paired to the Ph₃C⁺). For comparison, we consider the reactions

of anionic metal hydrides with alkyl halides (eq 10)^6,66as being

properly described by both terms. They are nucleophilic

displacements, since H^-displaces Br^-at the carbon center, but

they are also hydride transfers from the viewpoint of the metal

hydride. In other reactions of metal hydrides, the pair of

electrons in the M-H bond functions as a nucleophile, but

(58) Cheng, T.-Y.; Bullock, R. M., unpublished results.

(59) Sanders, J. R. J. Chem. Soc., Dalton Trans. 1973, 748-749.

(60) Gargano, M.; Giannoccaro, P.; Rossi, M.; Vasapollo, G.; Sacco, A.

J. Chem. Soc., Dalton Trans. 1975, 9-12.

(61) Bodner, G.; Gladysz, J. A.; Nielsen, M. F.; Parker, V. D. J. Am.

Chem. Soc. 1987, 109, 1757-1764.

(62) Hayes, J. C.; Cooper, N. J. J. Am. Chem. Soc. 1982, 104, 5570-

5572.

(63) Legzdins, P.; Martin, D. T. Inorg. Chem. 1979, 18, 1250-1254.

(64) Berke, H.; Burger, P. Comments Inorg. Chem. 1994, 16, 279-312.

(65) Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc. 1984, 106, 2554-

2558.

(66) Ash, C. E.; Hurd, P. W.; Darensbourg, M. Y.; Newcomb, M. J.

Am. Chem. Soc. 1987, 109, 3313-3317.

Conclusions

The kinetics of hydride transfer to Ph₃C⁺from a series of

neutral metal hydrides have been determined. For the complexes

reported here, the rate constants for kinetic hydricity span a

(67) Bullock, R. M.; Brammer, L.; Schultz, A. J.; Albinati, A.; Koetzle,

T. F. J. Am. Chem. Soc. 1992, 114, 5125-5130.

(68) Venanzi, L. M. Coord. Chem. ReV. 1982, 43, 251-274.

(69) Martin, B. D.; Warner, K. E.; Norton, J. R. J. Am. Chem. Soc. 1986,

108, 33-39.

13132 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

range of >10⁶. Most of these transition-metal hydrides are more

hydridic than the main group hydride HSiEt₃. Third-row metal

hydrides are more hydridic than their first row analogues. The

effect of changing metals (for a given set of ligands) is much

smaller than the effect of changing ligands on a given metal.

Complexes with Cp* ligands are more hydridic than those with

Cp ligands, indicating that electronic effects are more influential

than steric effects in determining the kinetic hydricity. Substitu-

tion of an electron-withdrawing CO ligand by an electron-

donating phosphine can result in a dramatic acceleration of

hydricitysthe rate constant for hydride transfer from trans-Cp-

(CO)₂(PMe₃)MoH exceeds that of Cp(CO)₃MoH by a factor of

10⁴. Steric effects partly counteract electronic effects on kinetic

hydricity in metal hydrides having a cis geometry of phosphine

and hydride ligands. The hydride transfers reported here are

thought to occur as single-step hydride transfers, rather than

electron transfer followed by hydrogen atom transfers. This

development of a scale of kinetic hydricity permits comparisons

with hydrogen atom transfer and proton-transfer reactions of

the same metal hydrides.

apparatus with an SU-40 spectrophotometer unit. Some slow kinetics

were performed on a Hewlett-Packard 8452A diode array spectropho-

tometer. The data were collected and analyzed on a Macintosh computer

with LabVIEW and MATLAB software. For each kinetics run, 300

data points were collected and analyzed.

Sample Preparation. The CH₂Cl₂solutions were prepared in a

glovebox under an argon atmosphere. For most experiments, the

solution of Ph₃C⁺BF₄^-was prepared as 0.1 mM and the metal hydride

(0.25 mmol) was used to make a 25-mL stock solution (10 mM), which

was then diluted to make 4 or 5 solutions. The metal hydride

concentrations were typically 10- to 100-fold in excess of the Ph₃C⁺

concentration. In the case of experiments with Cp(CO)₃WH, the

concentration of excess [Cp(CO)₃WH] was varied 50-fold.

Stopped-Flow Experiments. The flow circuit was flushed with dry,

oxygen-free CH₂Cl₂before the reagent solutions were used. One

reservoir syringe was then was filled with the Ph₃C⁺BF₄^-solution and

the other with the metal hydride solution. The two drive syringes were

flushed with the solutions twice before initiating the kinetic experiments

and collecting data. The rate of disappearance of the absorbance was

recorded at 450 nm (ꢀ ) 2.5 × 10⁴M^-1cm^-1, which was at the shoulder

of the absorption peak for Ph₃C⁺BF₄^-; at λ_max) 410 and 430 nm, ꢀ )

3.2 × 10⁴M^-1cm^-1). The average first-order rate constant was obtained

from at least 5 runs at each metal hydride concentration. Variation of

reproducibility from run to run, with use of the same solutions, was

typically <2%. This does not account for uncertainties in the concentra-

tion of [MH], due to weighing and dilution errors. As indicated in Table

1, we estimated an overall uncertainty of ≈10% for the rate constants,

based on reproducibility of rate data from duplicate runs with

independently prepared solutions. When the temperature of the constant-

temperature bath was changed, the solutions were allowed to equilibrate,

and the temperature of the bath was independently checked by using a

digital thermocouple.

Experimental Section

General. All manipulations were carried out under an atmosphere

of argon by using Schlenk or vacuum-line techniques, or in a Vacuum

Atmospheres drybox. ¹H NMR chemical shifts were referenced to the

residual proton peak of CD₂Cl₂at δ 5.32. Elemental analyses were

carried out by Schwarzkopf Microanalytical Laboratory (Woodside,

NY). NMR spectra were recorded on a Bruker AM-300 spectrometer

(300 MHz for ¹H). IR spectra were recorded on a Mattson Polaris FT-

IR spectrometer. Cp(CO)₃WH,⁷⁰Cp(CO)₃MoH,⁷⁰Cp*(CO)₃WH,⁷¹

Cp*(CO)₃MoH,⁷²Cp(CO)₂(PMe₃)MoH,²⁶Cp(CO)₂(PPh₃)MoH,⁷³

Cp*(CO)₃CrH,¹⁸cis-(CO)₄(PPh₃)ReH,⁷⁴the indenyl complex (η⁵-

UV Data for Substituted Trityl Cations in CH₂Cl₂. (a) Ph₂(p-

MeOC₆H₄)C⁺BF₄^-: λ_max) 398 nm (ꢀ ) 1.4 × 10⁴M^-1cm^-1) and

478 nm (ꢀ ) 5.3 × 10⁴M^-1cm^-1). (b) Ph(p-MeOC₆H₄)₂C⁺BF₄^-: λ_max

) 416 nm (ꢀ ) 3.6 × 10⁴M^-1cm^-1) and 504 nm (ꢀ ) 8.2 × 10⁴M^-1

cm^-1). (c) (p-MeOC₆H₄)₃C⁺BF₄^-: λ_max) 486 nm (ꢀ ) 8.9 × 10⁴M^-1

cm^-1).

C₉H₇)(CO)₃WH⁷⁵Cp(NO)₂WH,⁶³(CO)₅MnH,⁷⁶(CO)₅ReH,⁷⁷(CO)₄-

(PPh₃)MnH,⁷⁸Ph₃COTf,⁷⁹and Ph₃C⁺BAr′₄

were prepared by

-80

literature methods. Cp(CO)₃MoD and Cp*(CO)₃MoD were synthesized

from their corresponding metal hydrides, by exchange with CH₃OD

(99.5% D, Aldrich) at room temperature, as described previously⁸¹for

Preparation of Cp(CO)₂(PCy₃)MoH. A solution of Cp(CO)₃MoH

(1.00 g, 4.06 mmol) and PCy₃(1.12 g, 3.98 mmol) in toluene (30 mL)

was heated to 90-100 °C for 8 h. The solvent volume was reduced to

∼5 mL, and hexane (30 mL) was added. The precipitate was collected

by filtration and washed with hexane (20 mL × 3) to give Cp(CO)₂-

-

Cp(CO)₃MoD. Ph₃C⁺BF₄and Ph₃C⁺PF₆were purchased from

Aldrich and purified by recrystallization from CH₂Cl₂/ether. Ph₂(p-

MeOC₆H₄)C⁺BF₄^-, Ph(p-MeOC₆H₄)₂C⁺BF₄^-, and (p-MeOC₆H₄)₃C⁺-

BF₄were synthesized by a previously reported route,⁸²from the

-

1

(PCy₃)MoH (1.34 g, 2.69 mmol, 67%) as a pale yellow product. H

reactions of Ar₃CCl with HBF₄‚OEt₂. Ph(Me₂NC₆H₄)₂COH‚HCl was

purchased from Aldrich and (Me₂NC₆H₄)₃CCl (crystal violet) was

purchased from Fisher. PCy₃was purchased from Strem Chemicals

and used as received. HSiEt₃was distilled from LiAlH₄. THF, 1,2-

dimethoxyethane (DME), Et₂O, and hexane were distilled from Na/

benzopheneone, and CH₂Cl₂was distilled from P₂O₅.

NMR (CD₂Cl₂, 22 °C): δ 5.26 (s, 5 H, Cp), 1.00-1.95 [br, m, 33 H,

P(C₆H₁₁)₃], -6.13 (br, d, J_PH) 56.7 Hz, 1 H, MoH). ³¹P{¹H} NMR

(CD₂Cl₂, 22 °C): δ 76.2 (br, s). ¹³C{¹H} NMR (CD₂Cl₂, 22 °C): δ

1

242.1 (s, CO), 88.8 (s, C₅H₅), 39.1 (d, CH, J_PC) 21 Hz), 30.0 (s,

CH₂), 27.7 (d, CH₂, ³J_PC) 10 Hz), 26.5 (s, CH₂). IR (CH₂Cl₂): ν(CO)

1922 (vs), 1841 (s) cm^-1. Anal. Calcd for C₂₅H₃₉O₂PMo: C, 60.24; H,

7.89. Found: C, 59.80; H, 8.18.

Kinetic Experiments. Most of the kinetic measurements were

performed on a Hi-Tech Scientific SF-40 Canterbury stopped-flow

Isomerization Rates of Cp(CO)₂(PCy₃)MoH Measured by NMR

Line Broadening, (a) without Added [BF₄^-]. In the glovebox, Cp-

(CO)₂(PCy₃)MoH (12 mg, 0.024 mmol) in CD₂Cl₂(∼0.5 mL) was

prepared in an NMR tube with 1,2-dichloroethane as an internal line-

(70) Keppie, S. A.; Lappert, M. F. J. Chem. Soc. (A) 1971, 3216-3220.

(71) Kubas, G. J.; Kiss, G.; Hoff, C. D. Organometallics 1991, 10, 2870-

2876.

(72) Asdar, A.; Tudoret, M.-J.; Lapinte, C. J. Organomet. Chem. 1988,

349, 353-366.

1

width standard. The H NMR data were measured on a Bruker AM-

300 NMR instrument equipped with an Aspect 3000 computer and a

VT-1000 variable-temperature unit. The probe temperature was cali-

brated⁸³with MeOH (0.1% HCl) over the temperature range -86 to

27 °C. Line widths were determined by measuring the full width at

half-height of the hydride resonances (average line width of the doublet)

at seven different temperatures. The line widths in the absence of

exchange were 2.5 Hz. A Macintosh computer program was used to

simulate the line shape. This program was written by Dr. Soley S.

Kristja´nsdo´ttir; further details are provided in her Ph.D. thesis (Colorado

State University, 1991). The rate constants were determined by matching

computed line widths with experimental by determined ones. At -86

°C, both Cp and hydride resonances were sharp, showing no exchange

(73) Bainbridge, A.; Craig, P. J.; Green, M. J. Chem. Soc. (A) 1968,

2715-2718.

(74) Flitcroft, N.; Leach, J. M.; Hopton, F. J. J. Inorg. Nucl. Chem. 1970,

32, 137-143.

(75) Nesmeyanov, A. N.; Ustynyuk, N. A.; Makorova, L. G.; Andre, S.;

Ustynyuk, Y. A.; Novikova, L. N.; Luzikov, Y. N. J. Organomet. Chem.

1978, 154, 45-63.

(76) Bullock, R. M.; Rappoli, B. J. J. Organomet. Chem. 1992, 429,

345-368.

(77) Urbancic, M. A.; Shapley, J. R. Inorg. Synth. 1989, 26, 77-80.

(78) Booth, B. L.; Haszeldine, R. N. J. Chem. Soc. (A) 1966, 157-160.

(79) Straus, D. A.; Zhang, C.; Tilley, T. D. J. Organomet. Chem. 1989,

369, C13-C17.

(80) Bahr, S. R.; Boudjouk, P. J. Org. Chem. 1992, 57, 5545-5547.

(81) Sweany, R. L.; Comberrel, D. S.; Dombourian, M. F.; Peters, N.

A. J. Organomet. Chem. 1981, 216, 57-63.

1

of cis and trans isomers. H NMR (-86 °C): cis isomer, δ 5.24 (s,

(82) Olah, G. A.; Svoboda, J. J.; Olah, J. A. Synthesis 1972, 544.

(83) Van Geet, A. L. Anal. Chem. 1970, 42, 679-680.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13133

Cp), -6.25 (d, J_PH) 60.5 Hz, MoH); trans isomer, δ 5.10 (s, Cp),

-6.96 (d, J_PH) 20.0 Hz, MoH). The two hydride resonances started

to broaden at -45 °C and coalesced at -1 °C. The two Cp resonances

started to broaden at -68 °C and coalesced at -24 °C. The following

data are the line widths of the hydride resonances and the rate constants

for cisftrans isomerization (k_CT) at various temperatures: -45 °C, 3.0

Hz, 2.0 s^-1; -35 °C, 4.6 Hz, 6.5 s^-1; -24 °C, 9.3 Hz, 23 s^-1; -12 °C,

20.6 Hz, 65 s^-1; -1 °C, 31.8 Hz, 150 s^-1; 5 °C, 26.6 Hz, 200 s^-1; 11

°C, 19.7 Hz, 350 s^-1; 27 °C, 6.7 Hz, 1500 s^-1. The activation parameters

for the cis-Cp(CO)₂(PCy₃)MoH f trans-Cp(CO)₂(PCy₃)MoH isomer-

ization were ∆H^q) 11.6 ( 0.3 kcal mol^-1, ∆S^q) -5.6 ( 1.2 cal K^-1

mol^-1, and ∆G^q(25 °C) ) 13.3 kcal mol^-1(see Figure S9 in the

supporting information for an Eyring plot of these data). Using the

measured K_eq, we determined the activation parameters for the trans-

Cp(CO)₂(PCy₃)MoH f cis-Cp(CO)₂(PCy₃)MoH isomerization: ∆H^q

) 11.6 ( 0.3 kcal mol^-1, ∆S^q) -1.2 ( 1.2 cal K^-1mol^-1and ∆G^q

below:

total [BF₄^-]₀, mM

k_obs(s^-1

)

4.5

10

15

65

61

59

58

21

Kinetics of Cp(CO)₂(PCy₃)MoH with Excess Ph₃C⁺BF₄^-. Cp-

(CO)₂(PCy₃)MoH (5.0 mg, 0.010 mmol) was weighed in a 25-mL

volumetric flask and diluted to the mark with CH₂Cl₂to give a 0.40

mM solution. Ph₃C⁺BF₄^-(13.2 mg, 0.040 mmol) and NBu₄BF₄(90.2

mg, 0.274 mmol) were weighed in a 10-mL volumetric flask and diluted

to the mark with CH₂Cl₂to give a solution of [Ph₃C⁺] ) 4.0 mM with

total [BF₄^-] ) 31 mM. Another three concentrations of [Ph₃C⁺] (9,

-

18, and 27 mM) with constant total ionic strength of 31 mM [BF₄

]

(25 °C) ) 12.0 kcal mol^-1

.

were prepared. The concentrations of reagents for the reported results

below are half of those stated here since the volume is doubled during

mixing.

(b) With Added [BF₄^-]. An analogous procedure with the same

amount of Cp(CO)₂(PCy₃)MoH (12 mg, 0.024 mmol) as described

above was used, except that 0.11 M NBu₄BF₄(19 mg, 0.057 mmol,

2.4 equiv) was added to this NMR solution. The activation parameters

for the cis-Cp(CO)₂(PCy₃)MoH f trans-Cp(CO)₂(PCy₃)MoH isomer-

(a) At -55 °C. The observed rate constants for the reactions of

Cp(CO)₂(PCy₃)MoH (0.2 mM) with four [Ph₃C⁺BF₄^-] concentrations

(2, 4.5, 9, and 13.5 mM) were 0.58, 0.67, 0.73, and 0.84 s^-1

,

ization with 100 mM NBu₄BF₄were ∆H^q) 13.8 ( 0.2 kcal mol^-1

,

.

respectively. By fitting the equation, k_obs) {k_CTk_T/(k_TC+ k_T[Ph₃C⁺])

∆S^q) 1.9 ( 0.7 cal K^-1mol^-1, and ∆G^q(25 °C) ) 13.2 kcal mol^-1

+ k_C}[Ph₃C⁺] (where k_T) 1.9 × 10⁴M^-1s^-1, k_CT) 0.60 s^-1, k_TC

)

Thermodynamics for Cp(CO)₂(PR₃)MoH. The cis and trans

isomers were measured in CD₂Cl₂at several temperatures. The

equilibrium constant K was defined as [cis]/[trans]. The values of ∆H

and ∆S were obtained from plots of ln K vs T^-1. The percentages of

cis and trans isomers at 25 °C were obtained from the extrapolated

equilibrium constants.

5.4 s^-1), k_Cwas determined as 19 M^-1s^-1(see Figure 3).

(b) At -35, -15, and 15 °C. The k_obs, intercept, and slope (from

the line fit of 1/k_obsvs 1/[Ph₃C⁺BF₄^-]) are tabulated below.

[Ph₃C⁺BF₄

(mM)

]

k_obs

(s^-1

-

temp

(°C)

1

)

intercept (s)

1.2 × 10^-1

slope (s‚M)

3.8 × 10^-4

(a) Cp(CO)₂(PMe₃)MoH. H NMR spectra were recorded in the

1

range -86 to -1 °C. H NMR (-86 °C): cis (45%), δ 5.23 (s, Cp),

-35

-15

15

2.0

4.5

9.0

13

2.0

4.5

9.0

13

3.2

1.41 (d, J_PH) 9.5 Hz, PMe₃), -6.63 (d, J_PH) 65.7 Hz, MoH); trans

5.0

6.3

6.8

9.7

19

30

36

(55%), δ 5.06 (s, Cp), 1.46 (d, J_PH) 9.2 Hz, PMe₃), -6.46 (d, J_PH

)

23.1 Hz, MoH). The equilibrium constants were determined by

integration of the cyclopentadienyl resonances. The following equilib-

rium constants K_eqat different temperatures were found: -86 °C, 0.81;

-68 °C, 0.89; -56 °C, 0.93; -45 °C, 0.95; -35 °C, 0.99; -24 °C,

1.02; -12 °C, 1.07; -1 °C, 1.08. The thermodynamic data were ∆H

) 0.34 ( 0.02 kcal mol^-1, ∆S ) 1.4 ( 0.1 cal K^-1mol^-1, and ∆G (25

°C) ) -0.083 kcal mol^-1. At 25 °C, K_eqwas calculated as 1.15 (54%

cis, 46% trans).

1.4 × 10^-2

4.2 × 10^-4

1.8 × 10^-4

7.2 × 10^-5

2.0

4.5

9.0

13

27

64

120

157

(b) Cp(CO)₂(PPh₃)MoH. ¹H NMR spectra were recorded over the

1

range of -86 to -24 °C. H NMR (-86 °C): cis (55%), δ 5.13 (s,

Low-Temperature NMR Experiment of Cp(CO)₂(PPh₃)MoH

Cp), -5.44 (d, J_PH) 62.9 Hz, MoH); ³¹P NMR (-86 °C) for cis: δ

-

with Ph₃C⁺BF₄in CD₂Cl₂. Cp(CO)₂(PPh₃)MoH (12.0 mg, 0.025

1

72.6. NMR data for trans (45%), H NMR (-86 °C): δ 4.91 (s, Cp),

-

mmol) and Ph₃C⁺BF₄(8.2 mg, 0.025 mmol) were added to a 5-mm

-6.27 (d, J_PH) 21.2 Hz, MoH); ³¹P NMR (-86 °C) for trans: δ 70.6.

The equilibrium constants were determined by integration of the hydride

resonances. The following equilibrium constants K_eqwere found: -86

°C, 1.20; -68 °C, 1.27; -56 °C, 1.35; -45 °C, 1.40; -35 °C, 1.44;

-24 °C, 1.49. The thermodynamic data were ∆H ) 0.34 ( 0.01 kcal

mol^-1, ∆S ) 2.1 ( 0.1 cal K^-1mol^-1, and ∆G (25 °C) ) -0.30 kcal

mol^-1. At 25 °C, K_eq) 1.66, (62% cis, 38% trans).

NMR tube equipped with a Teflon J. Young valve. CD₂Cl₂(∼0.5 mL)

was added by vacuum transfer, and the solution was thawed at -78

°C. The tube was gently shaken, giving a color change from yellow to

red, and then quickly removed from the cold bath and inserted into a

1

precooled NMR probe (-80 °C). H NMR spectra showed the initial

formation of trans-Cp(CO)₂(PPh₃)MoFBF₃(90%) and [trans-Cp(CO)₂-

(PPh₃)Mo(ClCD₂Cl)]⁺[BF₄^-] (10%). The temperature of the NMR

probe was gradually increased. ¹H and ³¹P{¹H} NMR data and relative

yields are tabulated below.

(c) Cp(CO)₂(PCy₃)MoH. The cis and trans isomers were measured

at low temperatures (-86 to -45 °C) before the cyclopentadienyl (or

1

hydride) resonances became too flat to be integrated accurately. H

NMR (-86 °C): cis (89%), δ 5.24 (s, Cp), -6.25 (d, J_PH) 60.5 Hz,

MoH); trans (11%), δ 5.10 (s, Cp), -6.96 (d, J_PH) 20.0 Hz, MoH).

The percentages of two isomers were almost unchanged (changed <1%)

from -86 to -45 °C. At 22 °C, the ratio of cis:trans was 91:9, which

was calculated from the observed average phosphorus-hydride coupling

constant (J_PH) 56.7 Hz).

trans-MoFBF₃

temp (%); ¹H: δ Cp

(°C) ³¹P{¹H}: δ PPh₃

cis-MoFBF₃

(%); δ Cp

δ PPh₃

trans-Mo(ClCD₂Cl)⁺

(%); δ Cp

δ PPh₃

-80 90; 5.30 (br)

66.6 (d, J_PF) 19.9)

-30 88; 5.34 (d, J_PH) 1.9)

66.5 (s)

10; 5.39 (br)

59.3 (s)

10; 5.44 (d, J_PH) 1.9)

59.0 (s)

2; 5.64 (s)

50.3 (br)

52; 5.64 (s)

Effect of Ionic Strength on the Kinetics of Cp(CO)₂(PCy₃)MoH

with Excess Ph₃C⁺BF₄^-. Four solutions of Ph₃C⁺BF₄(9 mM) with

-

-8 45; 5.36 (d, J_PH) 2.1)

3; 5.47 (d, J_PH) 1.9)

added NBu₄BF₄(0, 11, 21, and 32 mM) in CH₂Cl₂were prepared in

CH₂Cl₂; a solution of Cp(CO)₂(PCy₃)MoH (1.2 mM in CH₂Cl₂) was

also prepared. The kinetics studies were performed at 15 °C; the

concentrations of reagents were half the initial ones because the volume

was doubled during mixing. The rate constants decrease with increasing

concentration of [BF₄^-]₀(total [BF₄^-] ) [Ph₃C⁺BF₄^-] + [NBu₄BF₄]),

but the effect is small (12% change over the range studied), as tabulated

66.5 (quintet, J_PF) 4.9) 50.2 (quintet, 58.8 (s)

J_PF) 8.4)

The sample in this NMR tube was then monitored at -8 °C. After

30 min, trans-Cp(CO)₂(PPh₃)MoFBF₃decreased to 16%, cis-Cp(CO)₂-

(PPh₃)MoFBF₃increased to 84%, and [trans-Cp(CO)₂(PPh₃)Mo(ClCD₂-

Cl)]⁺was no longer observed. After 1 h at -8 °C, only 5% of trans-

13134 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

Cp(CO)₂(PPh₃)Mo(FBF₃) remained and cis-Cp(CO)₂(PPh₃)MoFBF₃

increased to 95%.

An analogous reaction was carried out at 25 °C, after which the

NMR tube was cooled to at -80 °C. cis-Cp(CO)₂(PPh₃)MoFBF₃was

observed, with <1% trans-CpMo(CO)₂(PPh₃)FBF₃. ³¹P{¹H} NMR

Cl₂complex, 19%. The temperature was increased to 10 °C; after 20

min, the relative amounts were 31% for trans-CpMo(CO)₂(PMe₃)-

(FBF₃), 64% for cis-CpMo(CO)₂(PMe₃)(FBF₃), and 5% for [trans-Cp-

(CO)₂(PMe₃)Mo(ClCD₂Cl)]⁺. After 30 min, only 10% of trans-

CpMo(CO)₂(PMe₃)(FBF₃) remained, cis-CpMo(CO)₂(PMe₃)(FBF₃) had

increased to 90%, and [trans-Cp(CO)₂(PMe₃)Mo(ClCD₂Cl)]⁺was no

longer observed.

(CD₂Cl₂, -80 °C) of cis-Cp(CO)₂(PPh₃)MoFBF₃: δ 50.6 (d, J_PF

29.4 Hz).

)

NMR Tube Reaction of Cp(CO)₂(PPh₃)MoH with Ph₃COTf in

CD₂Cl₂. A solution of Cp(CO)₂(PPh₃)MoH (7.3 mg, 0.015 mmol) in

CD₂Cl₂(0.3 mL) was added to a solution of Ph₃COTf (6.0 mg, 0.015

mmol) in CD₂Cl₂(∼0.3 mL), giving a red solution. After 20 min, trans-

Cp(CO)₂(PPh₃)MoOTf and cis-Cp(CO)₂(PPh₃)MoOTf were observed

An analogous reaction was done at 25 °C, after which the solution

was cooled to -80 °C. The products ratio was ∼99:1 cis-Cp(CO)₂-

(PMe₃)MoFBF₃:trans-CpMo(CO)₂(PMe₃)MoFBF₃.

Low-Temperature NMR Experiment of Cp(CO)₂(PCy₃)MoH

with Ph₃C⁺BF₄^-. The NMR reaction was performed at -78 °C, as

described above, by using Cp(CO)₂(PCy₃)MoH (15.0 mg, 0.030 mmol)

and Ph₃C⁺BF₄^-(9.9 mg, 0.030 mmol). The tube was quickly removed

from the cold bath and inserted into a precooled NMR probe (-80

°C). A ³¹P{¹H} NMR (CD₂Cl₂, -80 °C) spectrum showed the major

peak at δ 51.2 (br), which was assigned to cis-Cp(CO)₂(PCy₃)MoFBF₃

(95%), and two small peaks at δ 62.0 (br) and δ 58.9 (s), which were

assigned to trans-Cp(CO)₂(PCy₃)MoFBF₃(4%) and [Cp(CO)₂-

(PCy₃)Mo(ClCD₂Cl)]⁺(1%). When temperature was increased to -40

°C, only cis-Cp(CO)₂(PCy₃)MoFBF₃[δ 51.3 (d, J_PF) 19.2 Hz)] was

observed.

1

by H NMR, in a ratio of 90:10. H NMR of trans-Cp(CO)₂(PPh₃)-

MoOTf: δ 5.29 (d, 5 H, Cp, J_PH) 2.2 Hz); cis-Cp(CO)₂(PPh₃)-

MoOTf: 5.62 (s, 5 H, Cp). ³¹P{¹H} NMR of trans-Cp(CO)₂(PPh₃)-

MoOTf: δ 66.0 (s); cis-Cp(CO)₂(PPh₃)MoOTf: 50.2 (s). The trans

complex slowly isomerized to cis complex. The following ratios were

recorded at various times: 6 h, trans:cis ) 55:45; 21 h, trans:cis )

42:58; 46 h, trans:cis ) 8:92. The equilibrium constant was not precisely

determined.

NMR Tube Reaction of Cp(CO)₂(PPh₃)MoH with Ph₃C⁺BF₄

-

in CD₃CN. Cp(CO)₂(PPh₃)MoH (7.3 mg, 0.015 mmol) in CD₃CN (0.3

mL) was added to a CD₃CN solution of Ph₃C⁺BF₄(5.0 mg, 0.015

-

NMR Tube Reaction of Cp(CO)₂(PPh₃)MoH with Ph₃C⁺BF₄

-

mmol). After 10 min, the NMR spectrum showed a 96:4 ratio of trans-

in CD₂Cl₂. A solution of Cp(CO)₂(PPh₃)MoH (7.3 mg, 0.015 mmol)

1

to cis-[CpMo(CO)₂(PPh₃)(CD₃CN)]⁺[BF₄]^-. H NMR: trans, δ 5.35

in CD₂Cl₂(0.3 mL) was added to a CD₂Cl₂solution of Ph₃C⁺BF₄

-

(d, Cp, J_PH) 1.8 Hz); cis, 5.65 (s, Cp). ³¹P{¹H} NMR: trans, δ 59.6

(s); cis, 52.4 (s).

(5.0 mg, 0.015 mmol) to give a red-purple solution of cis-Cp(CO)₂-

(PPh₃)MoFBF₃.^{84 1}H NMR (CD₂Cl₂): δ 7.60-7.30 (m, 15 H, PPh₃),

5.65 (s, 5 H, Cp). ³¹P{¹H} NMR (CD₂Cl₂): δ 50.0 (br, s). IR (CH₂-

NMR Tube Reaction of Cp(CO)₂(PPh₃)MoH with Ph₃COTf in

CD₃CN. A solution of Cp(CO)₂(PPh₃)MoH (7.3 mg, 0.015 mmol) in

CD₃CN (0.3 mL) was added to a solution of Ph₃COTf (6.0 mg, 0.015

mmol) in CD₃CN (0.3 mL) to give a yellow solution. The NMR

spectrum indicated a trans:cis ratio of 96:4 for [CpMo(CO)₂(PPh₃)-

(CD₃CN)]⁺[OTf]^-. ¹H NMR: trans, δ 5.36 (d, Cp, J_PH) 1.8 Hz); cis,

Cl₂): ν(CO) 1989 (vs), 1905 (s) cm^-1

.

-

NMR Tube Reaction of Cp(CO)₂(PMe₃)MoH with Ph₃C⁺BF₄

in CD₂Cl₂. A procedure analogous to that described above gave cis-

Cp(CO)₂(PMe₃)MoFBF₃.^{85 1}H NMR (CD₂Cl₂): δ 5.66 (s, 5 H, Cp),

1.60 (d, PMe₃, J_PH) 10.3 Hz). ³¹P{¹H} NMR (CD₂Cl₂): δ 12.4 (br).

5.66 (s, Cp). ³¹P{¹H} NMR: trans, δ 59.7 (s); cis, 52.7 (s).

IR (CH₂Cl₂): ν(CO) 1989 (s), 1898 (s) cm^-1

.

-

NMR Tube Reaction of Cp(CO)₂(PCy₃)MoH with Ph₃C⁺BF₄

,

-

Kinetics for trans-Cp(CO)₂(PR₃)MoH. The k_Hrate constants were

obtained from plots of k_obsvs [trans-Cp(CO)₂(PR₃)MoH]. The con-

centrations of trans isomers were corrected for the small temperature

dependence of the equilibrium constant K_eq.

(a) in CD₂Cl₂. A solution of Cp(CO)₂(PCy₃)MoH (9.5 mg, 0.019 mmol)

in CD₂Cl₂(0.3 mL) was added to a CD₂Cl₂solution of Ph₃C⁺BF₄

-

(6.3 mg, 0.019 mmol). An NMR spectrum recorded after 10 min showed

only cis-Cp(CO)₂(PCy₃)MoFBF₃. ¹H NMR: δ 5.75 (s). ³¹P{¹H}

NMR: δ 51.5 (s).

Kinetic Measurements by Conventional Methods for Slow

Reactions. Some reactions that were relatively slow for the stopped-

flow experiment were measured by conventional methods on a UV-

vis spectrophotometer. The reagent solutions were prepared in the

glovebox. In an H-shaped apparatus containing one 1.00-cm UV-vis

spectrophotometer cell and one tube (and both sides equipped with

Teflon valves), 2.0 mL (volumetric pipet) of the Ph₃C⁺solution was

placed in one side and 2.0 mL (volumetric pipet) of the metal hydride

solution was placed in the other side. The H-shaped apparatus was

removed from the glovebox and immersed into a constant-temperature

bath (25 °C) for 5 min. After the temperature was equilibrated, the

two solutions were mixed by shaking vigorously, and the UV-vis cell

containing the mixture was quickly placed in a cell holder in a Hewlett-

Packard 8452A diode-array spectrophotometer. The cell holder of the

spectrometer was thermostated at 25 °C by using a recirculating

constant-temperature bath. Spectra were recorded for ∼10 half-lives.

(b) in CD₃CN. The analogous procedure was performed except that

CD₃CN was used. The NMR spectra indicated a trans:cis ratio of 16:

1

84 for [CpMo(CO)₂(PCy₃)(CD₃CN)]⁺[BF₄]^-. H NMR: trans, δ 5.57

(d, Cp, J_PH) 1.2 Hz); cis, 5.71 (s, Cp). ³¹P{¹H} NMR: trans, δ 58.6

(s); cis, 49.9 (s).

Low-Temperature NMR Experiment of Cp(CO)₂(PMe₃)MoH

with Ph₃C⁺BF₄^-in CD₂Cl₂. The NMR reaction was performed at -78

°C as described above, with Cp(CO)₂(PMe₃)MoH (7.5 mg, 0.025 mmol)

and Ph₃C⁺BF₄(8.4 mg, 0.025 mmol). The tube was then quickly

-

removed from the cold bath and inserted into a precooled NMR probe

(-80 °C). The ¹H and ³¹P{¹H} NMR data and relative yields are

tabulated below.

trans-MoFBF₃

(%); ¹H: δ Cp

cis-MoFBF₃

(%); δ Cp

δ CH₃

trans-Mo(ClCD₂Cl)⁺

(%); δ Cp

temp δ CH₃

δ CH₃

Preparation of CpW(CO)₃FBF₃. CpW(CO)₃FBF₃was synthesized

(°C) ³¹P{¹H}: δ PMe₃

δ PMe₃

and isolated by a published method^21,86from the reaction of Cp-

(CO)₃WH with Ph₃C⁺BF₄in CH₂Cl₂at -40 °C. H NMR (CD₂Cl₂,

22 °C): δ 6.10 (s, 5 H, Cp). IR (CH₂Cl₂): ν(CO) 2067 (s), 1975 (vs)

cm^-1. Visible region in CH₂Cl₂: λ_max) 500 nm, ꢀ ) 5.9 × 10²M^-1

-

1

-80 65; 5.42 (br)

1.49 (d, J_PH) 9.6 Hz)

26.2 (d, J_PF) 20.5)

-60 61; 5.43 (br)

1.50 (d, J_PH) 9.9 Hz)

25.8 (d, J_PF) 20.7)

-40 57; 5.44 (br)

1.51 (d, J_PH) 9.9 Hz)

25.3 (m br)

9; 5.65 (s)

26; 5.52 (br)

1.53 (d, J_PH) 10.5 Hz) 1.64 (d, J_PH) 10.0 Hz)

15.09 (m br)

8; 5.63 (s)

1.54 (d, J_PH) 10.5 Hz) 1.65 (d, J_PH) 10.3 Hz)

14.5 (quintet, J_PF) 9.5) 23.0 (s)

5; 5.64 (s)

1.56 (d, J_PH) 10.4 Hz) 1.67 (d, J_PH) 10.3 Hz)

14.0 (quintet, J_PF) 9.5) 22.6 (s)

8; 5.65 (s)

23.41 (s)

31; 5.53 (br)

cm^-1

.

Preparation of Cp(CO)₃MoFBF₃. Cp(CO)₃MoFBF₃was synthe-

sized and isolated by a published method^21,86from the reaction of Cp-

(CO)₃MoH with Ph₃C⁺BF₄^-in CH₂Cl₂at -40 °C. ¹H NMR (CD₂Cl₂,

22 °C): δ 5.96 (s, 5 H, Cp). IR (CH₂Cl₂): ν(CO) 2076 (s), 1995 (vs)

38; 5.55 (br)

-10 72; 5.45 (d, J_PH) 2.3)

1.53 (d, J_PH) 9.9 Hz)

20; 5.57 (d, J_PH) 2.1)

cm^-1

.

1.58 (d, J_PH) 10.4 Hz) 1.69 (d, J_PH) 10.3 Hz)

24.9 (quintet, J_PF) 5.2) 13.2 (quintet, J_PF) 9.5) 22.0 (s)

(84) Su¨nkel, K.; Ernst, H.; Beck, W. Z. Naturforsch. 1981, 36b, 474-

481.

(85) Appel, M.; Beck, W. J. Organomet. Chem. 1987, 319, C1-C4.

(86) Beck, W.; Schloter, K.; Su¨nkel, K.; Urban, G. Inorg. Synth. 1989,

26, 96-106.

The NMR spectrum was then monitored at -10 °C for 45 min. The

percentages of the three complexes changed only slightly: trans-CpMo-

(CO)₂(PMe₃)(FBF₃), 69%; cis-CpMo(CO)₂(PMe₃)(FBF₃), 12%; CD₂-

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13135

Preparation of Cp*(CO)₃WFBF₃. A solution of Cp*(CO)₃WH (270

mg, 0.666 mmol) in CH₂Cl₂(8 mL) was transferred by cannula to a

solution of Ph₃C⁺BF₄^-(200 mg, 0.606 mmol) in CH₂Cl₂(10 mL) that

had been cooled to -40 °C. The solution immediately turned red and

then violet. After stirring the solution at -40 °C for 20 min, the solvent

was reduced to ∼5 mL and hexane (∼20 mL) was added by vacuum

transfer to precipitate the product; the product was collected by filtration,

washed with hexane (5 mL × 3), and dried under vacuum to give Cp*-

NMR Tube Reaction of Cp*(CO)₃CrH with Ph₃C⁺BF₄^-. A

solution of Cp*(CO)₃CrH (8.5 mg, 0.031 mmol) in CD₂Cl₂(0.3 mL)

was added to a CD₂Cl₂solution of Ph₃C⁺BF₄^-(10.3 mg, 0.031 mmol)

in CD₂Cl₂. The reaction mixture turned deep green immediately and

then black. ¹H NMR indicated that one equivalent of Ph₃CH was

formed, but the initially formed metal complex decomposed to

unidentified products.

NMR Tube Reaction of Cp(NO)₂WH with Ph₃C⁺BF₄^-. Cp(NO)₂-

WH (5.3 mg, 0.017 mmol) was dissolved in CD₂Cl₂(∼0.3 mL) to give

a bright green solution, which was added to a yellow CD₂Cl₂solution

1

(CO)₃WFBF₃(250 mg, 0.51 mmol, 84%) as a purple solid. H NMR

(CD₂Cl₂, 22 °C): δ 2.10 (s, 15 H, Cp*). ¹³C{¹H} NMR (CD₂Cl₂, 22

1

of Ph₃C⁺BF₄(5.6 mg, 0.017 mmol). The reaction mixture turned

-

°C): δ 232.8 (s, 1 CO, J_CW) 134 Hz), 229.6 (s, 2 CO J_CW) 164

Hz), 109.6 (s, C₅Me₅), 10.6 (s, C₅Me₅). IR (CH₂Cl₂): ν(CO) 2050 (s),

1959 (vs) cm^-1. Visible region in CH₂Cl₂: λ_max) 502 nm, ꢀ ) 6.0 ×

10²M^-1cm^-1. Anal. Calcd for C₁₃H₁₅BF₄O₃W: C, 31.87; H, 3.09.

Found: C, 31.76; H, 3.25.

darker green immediately. The ¹H NMR spectrum of the resulting

solution showed that Cp(NO)₂WH was completely consumed and two

metal complexes were formed, with Cp resonances at δ 6.37 and δ

6.28 (relative integration ∼1:1.2), which we assigned to {[CpW(NO)₂]₂-

(µ-H)}⁺BF₄^-[δ 6.37 (s, 10 H, Cp), -8.60 (s, 1 H, W₂H, J_WWH) 113

Hz)]²⁰and CpW(NO)₂(FBF₃) (δ 6.28, s, 5 H, Cp),⁹¹respectively.

Preparation of cis-Cp(CO)₂(PCy₃)MoFBF₃. A solution of Cp(CO)₂-

(PCy₃)MoH (503 mg, 1.01 mmol) in CH₂Cl₂(15 mL) was added to a

-

solution of Ph₃C⁺BF₄(315 mg, 0.96 mmol) in CH₂Cl₂(10 mL) that

-

Reaction of (η⁶-C₆Me₆)(CO)₂MnH with Ph₃C⁺BF₄in CH₃CN.

(η⁶-C₆Me₆)(CO)₂MnH (2.7 mg, 0.010 mmol) was dissolved in CH₃-

had been cooled to -40 °C. The solution turned dark brown im-

mediately and then violet when the addition was completed. The

solution was stirred at -40 °C for 40 min, after which the solvent was

reduced to ∼5 mL and hexane (25 mL) was added. The mixture was

kept at -78 °C for 3 h and then filtered while cold. The precipitate

was collected by filtration, washed with hexane (5 mL × 2), and dried

under vacuum to give Cp(CO)₂(PCy₃)MoFBF₃(477 mg, 0.82 mmol,

85%) as a violet solid. ¹H NMR (CD₂Cl₂, 22 °C): δ 5.75 (s, 5 H, Cp),

1.25-2.70 [br, m, 33 H, P(C₆H₁₁)₃]. ³¹P{¹H} NMR (CD₂Cl₂, 22 °C):

δ 51.5 (s). ¹³C{¹H} NMR (CD₂Cl₂, 22 °C): δ 256.1 (d, CO, ²J_CP) 24

-

CN (∼0.5 mL). Ph₃C⁺BF₄(3.3 mg, 0.010 mmol) was dissolved in

CH₃CN (0.5 mL), and the two solutions were mixed. The solvent was

1

evaporated and the residue was redissolved in CD₂Cl₂. The H NMR

1

showed only [(η⁶-C₆Me₆)(CO)₂Mn(CH₃CN)]⁺[BF₄]^-and Ph₃CH. H

NMR (CD₂Cl₂) of [(η⁶-C₆Me₆)(CO)₂Mn(NCCH₃)]⁺[BF₄]^-: δ 2.38 (s,

3 H, CH₃CN), 2.28 (s, 18 H, C₆Me₆). IR (CD₂Cl₂): ν(CO) 2002 (vs),

1954 (vs) cm^-1

.

Competition Reaction of (η⁶-C₆Me₆)(CO)₂MnH and Cp*-

(CO)₃WH with Ph₃C⁺BF₄^-. (η⁶-C₆Me₆)(CO)₂MnH (5.5 mg, 0.020

mmol, 2 equiv) and Cp*(CO)₃WH (8.1 mg, 0.020 mmol, 2 equiv) were

dissolved in CD₃CN (∼0.4 mL) in an NMR tube. After the initial

1

Hz), 251.4 (s, CO), 95.5 (s, C₅H₅), 34.7 (d, CH, J_CP) 17 Hz), 29.7

(d, CH₂, ²J_CP) 55 Hz), 27.7 (d, CH₂, ³J_CP) 10 Hz), 26.5 (s, CH₂). IR

(CH₂Cl₂): ν(CO) 1976 (vs), 1893 (s) cm^-1. Visible region in CH₂Cl₂:

λ_max) 516 nm, ꢀ ) 6.6 × 10²M^-1cm^-1. Anal. Calcd for C₂₅H₃₈-

BF₄O₂PMo: C, 51.39; H, 6.56. Found: C, 51.40; H, 6.94.

spectrum was taken, this solution was added to a solution of Ph₃C⁺BF₄

-

(3.3 mg, 0.010 mmol, 1 equiv) in CD₃CN (∼0.1 mL). The reaction

1

mixture displayed a H NMR spectrum that showed the formation of

NMR Tube Reaction of Cp*(CO)₃MoH with Ph₃C⁺BF₄in

-

[(η⁶-C₆Me₆)(CO)₂Mn(CD₃CN)⁺] (δ 2.23, C₆Me₆) and [Cp*W(CO)₃-

(CD₃CN)⁺] (δ 2.13, C₅Me₅) in a ratio of 4:1, with the remaining

amounts of (η⁶-C₆Me₆)(CO)₂MnH and Cp*(CO)₃WH agreeing with the

product formation.

CD₂Cl₂. A procedure analogous to that described above gave Cp*-

(CO)₃MoFBF₃.^{87 1}H NMR (CD₂Cl₂): δ 1.98 (s, 15 H, Cp*). IR (CD₂-

Cl₂): ν(CO) 2056 (s), 1974 (vs) cm^-1

.

NMR Tube Reaction of (CO)₄(PPh₃)MnH with Ph₃C⁺BF₄in

CD₂Cl₂. A procedure analogous to that described above gave cis-(CO)₄-

(PPh₃)Mn(FBF₃).^{88 1}H NMR (CD₂Cl₂): δ 7.65-7.36 (m, 15 H, PPh₃).

³¹P{¹H} NMR (CD₂Cl₂): δ 42.1 (br, s). IR (CD₂Cl₂): ν(CO) 2114

-

Preparation of Na(C₅H₄CO₂Me)W(CO)₃‚DME.⁹²A solution of

W(CO)₆(7.17 g, 20.38 mmol) and Na(C₅H₄CO₂Me)⁹³(3.32 g, 22.7

mmol) was refluxed in DME (60 mL) for 2 days. The mixture was

cooled and filtered to remove some green-gray insoluble material and

unreacted W(CO)₆. The solvent was evaporated, and the sticky orange

residue was washed with ether (100 mL) to give Na(C₅H₄CO₂Me)-

(CO)₃W‚DME (5.24 g, 10.39 mmol, 51% yield) as a yellow solid. After

drying under vacuum for 2 days, the product still contained 1 equiv of

DME. IR (CH₂Cl₂): ν(CO) 1904 (s), 1805 (s), 1761 (s) cm^-1. ¹H NMR

(CD₂Cl₂): δ 5.64 (t, 2 H, H_2,5, J_HH) 2.4 Hz), 5.10 (t, 2 H, H_3,4, J_HH

) 2.4 Hz), 3.62 (s, 3 H, CH₃), 3.45 (s, 4 H, CH₂of DME), 3.28 (s, 6

H, CH₃of DME).

Preparation of (C₅H₄CO₂Me)(CO)₃WH.⁹²CF₃COOH (0.37 mL,

4.80 mmol) was added to a solution of Na(C₅H₄CO₂Me)(CO)₃W‚DME

(1.90 g, 3.97 mmol) in THF (60 mL) at -78 °C. The reaction mixture

immediately turned yellow. The solvent was evaporated, and the

resulting grayish-yellow residue was extracted with hexane (70 mL).

The light-yellow solution was concentrated to 5 mL and cooled to -78

°C for 10 min. The precipitate was collected by filtration and dried

under vacuum to give (C₅H₄CO₂Me)(CO)₃WH (885 mg, 2.38 mmol,

60% yield) as a pale yellow solid. (More product can be isolated by

extracting the residue with diethyl ether, followed by adding hexane,

and concentrating the solution mixture; however, the darker yellow

solids obtained need to be purified by sublimation at 60 °C). ¹H NMR

(m), 2044 (sh), 2028 (s), 1977 (s) cm^-1

.

NMR Tube Reaction of (CO)₄(PPh₃)ReH with Ph₃C⁺BF₄in

CD₂Cl₂. A procedure analogous to that described above gave cis-(CO)₄-

(PPh₃)Re(FBF₃).^{88 1}H NMR (CD₂Cl₂): δ 7.63-7.42 (m, 15 H, PPh₃).

³¹P{¹H} NMR (22 °C): δ 16.0 (quintet, J_PF) 7.3 Hz). IR (CH₂Cl₂):

-

ν(CO) 2121 (m), 2021 (vs), 1964 (s) cm^-1

.

NMR Tube Reaction of (η⁵-C₉H₇)(CO)₃WH with Ph₃C⁺BF₄^-in

CD₂Cl₂. A procedure analogous to that described above gave (η⁵-C₉H₇)-

(CO)₃W(FBF₃).^{21 1}H NMR (CD₂Cl₂): δ 7.77-7.74 (m, 2 H), 7.66-

7.61 (m, 2H), 6.15 (d, J ) 3 Hz, 2 H), 6.10 (m, 1H). IR (CH₂Cl₂):

ν(CO) 2062 (s), 1977 (vs) cm^-1

.

Reaction of (CO)₅MnH with Ph₃C⁺BF₄in CH₂Cl₂. (CO)₅MnH

(3.0 mg, 0.015 mmol) was weighed in a microsyringe and then dissolved

in CH₂Cl₂(0.5 mL). This solution was mixed with a solution of

Ph₃C⁺BF₄^-(5.0 mg, 0.015 mmol) in CH₂Cl₂(0.5 mL). Clean formation

of (CO)₅MnFBF₃was observed by IR. This product has been previously

prepared⁸⁹by reacting CH₃Mn(CO)₅with Ph₃C⁺BF₄^-. IR (CH₂Cl₂):

-

ν(CO) 2073 (vs), 2016 (m) cm^-1

Reaction of (CO)₅ReH with Ph₃C⁺BF₄in CH₂Cl₂. A procedure

.

-

analogous to that used for the Mn analogue was performed, giving

(CD₂Cl₂, 22 °C): δ 6.02 (t, 2 H, H_2,5, J_HH) 2.3 Hz), 5.57 (t, 2 H, H_3,4

,

Re(CO)₅FBF₃.⁹⁰IR (CH₂Cl₂): ν(CO) 2057 (vs), 2002 (m) cm^-1

.

J_HH) 2.3 Hz), 3.78 (s, 3 H, CH₃), -7.07 (s, 1 H, WH, J_WH) 36.5

Hz). ¹³C{¹H} NMR (CD₂Cl₂, 22 °C): δ 215.2 (br s, CO), 165.0 (s,

COO), 95.4 (s, CCOO), 92.2 (s, C_2,5H), 90.4 (s, C_3,4H), 52.5 (s, CH₃).

(87) Leoni, P.; Auquilini, E.; Pasquali, M.; Marchetti, F.; Sabat, M. J.

Chem. Soc., Dalton Trans. 1988, 329-333.

(88) Schweiger, M.; Beck, W. Z. Anorg. Allg. Chem. 1991, 595, 203-

210.

(89) Raab, K.; Nagel, U.; Beck, W. Z. Naturforsch. 1983, 38b, 1466-

1476.

(91) Legzdins, P.; Martin, D. T. Organometallics 1983, 2, 1785-1791.

(92) Hart, W. P.; Macomber, D. W.; Rausch, M. D. J. Am. Chem. Soc.

1980, 102, 1196-1198.

(93) Hart, W. P.; Shihua, D.; Rausch, M. D. J. Organomet. Chem. 1985,

282, 111-121.

(90) Raab, K.; Olgemo¨ller, B.; Schloter, K.; Beck, W. J. Organomet.

Chem. 1981, 214, 81-86.

13136 J. Am. Chem. Soc., Vol. 120, No. 50, 1998

Cheng et al.

IR (CH₂Cl₂): ν(CO) 2028 (s), 1935 (vs), 1730 (m) cm^-1. Anal. Calcd

for C₁₀H₈O₅W: C, 30.64; H, 2.06. Found: C, 30.79; H, 2.03.

Preparation of (C₅H₄CO₂Me)(CO)₃WFBF₃. A solution of (C₅H₄-

CO₂Me)(CO)₃WH (374 mg, 0.95 mmol) in CH₂Cl₂(15 mL) was added

(∼0.3 mL) and was added to a CD₂Cl₂solution of Ph₃C⁺BAr′₄^-(16.6

1

mg, 0.015 mmol). The H NMR spectrum recorded in 5 min showed

a major Cp resonance at δ 6.12, tentatively assigned to [Cp(CO)₃-

W(ClCD₂Cl)]⁺BAr′₄^-. After 1 day, the peak at δ 6.12 was converted

to a new (unidentified) resonance at δ 5.88.

to a solution of Ph₃C⁺BF₄(300 mg, 0.91 mmol) in CH₂Cl₂(10 mL)

-

at -30 °C. The resulting red solution was warmed to room temperature

and stirred for 70 min. The solvent was reduced to 5 mL, hexane (25

mL) was added, and the precipitates were collected by filtration, washed

with hexane (5 mL × 2), and dried under vacuum to give (C₅H₄CO₂-

Me)(CO)₃WFBF₃(370 mg, 0.77 mmol, 85% yield) as maroon solid.

¹H NMR (CD₂Cl₂, 22 °C): δ 6.39 (t, 2 H, H_2,5, J_HH) 2.4 Hz), 6.30 (t,

2 H, H_3,4, J_HH) 2.4 Hz), 3.84 (s, 3 H, CH₃). ¹³C{¹H} NMR (CD₂Cl₂,

22 °C): δ 225.9 (s, 1 CO), 221.4 (s, 2 CO), 164.0 (s, COO), 101.3 (s,

Appendix

Analysis of the Kinetics Shown in Scheme 2, Involving

Cis-Trans Equilibration of MH and Hydride Transfer from

Both Isomers to Ph₃C⁺

When the cis/trans equilibrium of MH favors the cis isomer,

we can apply the steady-state approximation to [trans-MH]:

C

_2,5H), 96.9 (s, CCOO), 95.1 (s, C_3,4H), 53.4 (s, CH₃). IR (CH₂Cl₂):

ν(CO) 2074 (s), 1990 (vs), 1736 (m) cm^-1. Anal. Calcd for C₁₀H₇-

BF₄O₅W: C, 25.14; H, 1.48. Found: C, 25.72; H, 2.00.

d[trans-MH]

) k_CT[cis-MH] - k_TC[trans-MT] -

dt

Preparation of Ph(Me₂NC₆H₄)₂C⁺BF₄^-. An aqueous solution (100

mL) of Ph(Me₂NC₆H₄)₂COH‚HCl (500 mg, 1.31 mmol) was mixed

with an aqueous solution (5 mL) of NH₄BF₄(151 mg, 1.44 mmol).

The product precipitated as fine blue-green solids and was collected

k_T[trans-MH] [Ph₃C⁺] ) 0 (A1)

k_CT[cis-MH]

[trans-MH] )

(A2)

k_TC+ k_T[Ph₃C⁺]

1

by filtration and recrystallized from CH₂Cl₂/Et₂O. H NMR (CD₂Cl₂,

22 °C): δ 7.73 (apparent t, 1 H, J_HH) 7.4 Hz), 7.57 (apparent t, 2 H,

J_HH) 7.4 Hz), 7.43-7.34 (m, 6H), 6.91 (d, 4 H, J_HH) 9.3 Hz), 3.32

(s, 12 H, CH₃).

- d[cis-MH]

dt

) k_CT[cis-MH] - k_TC[trans-MH] +

k_C[cis-MH][Ph₃C⁺] (A3)

Preparation of (Me₂NC₆H₄)₃C⁺BF₄^-. An aqueous solution (20 mL)

of (Me₂NC₆H₄)₃CCl (crystal violet; 600 mg, 1.47 mmol) was mixed

with an aqueous solution (5 mL) of NH₄BF₄(154 mg, 1.47 mmol).

The product precipitated as a fine purple solid and was collected by

Substitution of [trans-MH] into this expression for - d[cis-

MH]/dt, followed by algebraic manipulation, gives

1

filtration and recrystallized from CH₂Cl₂/Et₂O. H NMR (CD₂Cl₂): δ

7.35 (d, 6 H, C₆H₄NMe₂, J_HH) 9.2 Hz), 6.85 (d, 6 H, C₆H₄NMe₂, J_HH

) 9.2 Hz), 3.23 (s, 18 H, CH₃).

k_CTk_T

k_TC+ k_T[Ph₃C⁺]

- d[cis-MH]

dt

)

+ k_C[Ph₃C⁺][cis-MH]

Reaction of Cp(CO)₂(PMe₃)MoH with Ph(Me₂NC₆H₄)₂C⁺BF₄

.

-

(

)

A solution of Cp(CO)₂(PMe₃)MoH (7.5 mg, 0.025 mmol) in CD₂Cl₂

(A4)

(0.3 mL) was added to a solution of Ph(Me₂NC₆H₄)₂C⁺BF₄^-(10.6 mg,

1

0.025 mmol) in CD₂Cl₂(0.3 mL). After 1 h, H NMR indicated that

Note that the rate of disappearance of cis-MH is equal to the

rate of disappearance of [Ph₃C⁺] and the rate of appearance of

product:

∼67% of Ph(Me₂NC₆H₄)₂CBF₄was converted to Ph(Me₂NC₆H₄)₂CH.

Several unidentified metal products were formed, but CpMo(CO)₂-

(PMe₃)(FBF₃) was not observed.

Reaction of Cp(CO)₂(PMe₃)MoH with (Me₂NC₆H₄)₃C⁺BF₄^-. An

procedure analogous to that described above was used. After 2 days,

only 6% of (Me₂NC₆H₄)₃CH was formed, and no Cp(CO)₂(PMe₃)-

MoFBF₃was observed.

- d[Ph₃C⁺]

- d[cis-MH]

dt

d[product]

)

(A5)

dt

Preparation of (C₅H₄Me)(CO)₃WH. (C₅H₄Me)(CO)₃WH was

Three cases encountered in this work are described below:

1. For experiments using Cp(CO)₂(PCy₃)MoH with [Ph₃C⁺]

. [MH], the minor isomer, trans-MH (11%), reacts much faster

than cis-MH. The initial part of the reaction is too fast to be

observed in our stopped-flow experiments. After this initial

reaction, [trans-MH] reaches a steady-state concentration that

does not significantly change during the rest of the reaction.

The steady-state approximation is then valid, and we observe

this part of the reaction. The rate is pseudo-first-order, with

synthesized analogously to Cp(CO)₃WH⁷⁰but with use of C₅H₄Me

1

instead of C₅H₆. The product was isolated as an orange oil. H NMR

(CD₂Cl₂): δ 5.50 (t, 2 H, CH, J_HH) 2.2 Hz), 5.34 (t, 2 H, CH, J_HH

)

2.2 Hz), 2.20 (s, 3 H, CH₃), -7.17 (s, 1 H, WH, J_WH) 37.5 Hz). IR

(hexane): ν(CO) 2023 (s), 1935 (vs) cm^-1

.

NMR Tube Reaction of (C₅H₄Me)(CO)₃WH with Ph₃C⁺BF₄^-in

CD₂Cl₂. ¹H NMR (CD₂Cl₂) for (C₅H₄Me)W(CO)₃(FBF₃): δ 6.17 (t, 2

H, CH, J_HH) 2.0 Hz), 5.52 (t, 2 H, CH, J_HH) 2.0 Hz), 1.91 (s, 3 H,

CH₃). IR (CD₂Cl₂): ν(CO): 2063 (s), 1973 (vs) cm^-1

.

Preparation of (CO)₄(PCy₃)MnH. (CO)₄(PCy₃)MnH was first

reported by Hieber and co-workers.⁹⁴We prepared it from reaction of

(CO)₅MnH with PCy₃, using a procedure analogous to that for

preparation of (CO)₄(PPh₃)MnH.^{78 1}H NMR (CD₂Cl₂, 22 °C): δ 2.04-

1.18 [br m, 33 H, P(C₆H₁₁)₃], -7.91 (d, 1 H, MnH, J_PH) 34.5 Hz).

³¹P{¹H} NMR (CD₂Cl₂, 22 °C): δ 70.9 (s).

- d[cis-MH] d[product]

)

) k_obs[cis-MH] (A6)

dt

where

-

NMR Tube Reaction of (CO)₄(PCy₃)MnH with Ph₃C⁺BF₄in

k_CTk_T

k_TC+ k_T[Ph₃C⁺]

k_obs

)

+ k_C[Ph₃C⁺]

(A7)

CD₂Cl₂. ¹H NMR (22 °C) for (CO)₄(PCy₃)MnFBF₃: δ 2.40-1.14 [br

m, 33 H, P(C₆H₁₁)₃] ³¹P{¹H} NMR (CD₂Cl₂, 22 °C): δ 54.5 (s).

NMR Tube Reaction of Cp(CO)₂(PCy₃)MoH with Ph₃COTf in

CD₂Cl₂. The analogous procedure described above was used. The NMR

spectra were recorded immediately and only cis-Cp(CO)₂(PCy₃)MoOTf

(

)

2. For experiments using Cp(CO)₂(PR₃)MoH with [MH] .

[Ph₃C⁺], if the cis/trans equilibration is fast and k_TC

.

1

was observed. H NMR: δ 5.68 (s, 5 H, Cp). ³¹P{¹H} NMR (22 °C):

k_T[Ph₃C⁺], then a pseudo-first-order decay is again observed,

and the steady-state approximation is valid. These conditions

are applicable for our experiments with excess Cp(CO)₂(PCy₃)-

MoH (at all conditions investigated, from -20 to 25 °C) and

for the determination of k_Tof trans-Cp(CO)₂(PPh₃)MoH at 25

°C, the only temperature at which that was studied. From eq

δ 51.0 (s).

-

NMR Tube Reaction of Cp(CO)₃WH with Ph₃C⁺BAr′₄in

CD₂Cl₂. Cp(CO)₃WH (5.0 mg, 0.015 mmol) was dissolved in CD₂Cl₂

(94) Hieber, W.; Faulhaber, G.; Theubert, F. Z. Anorg. Allg. Chem. 1962,

314, 125-143.

Kinetic Hydricity of Metal Carbonyl Hydrides

J. Am. Chem. Soc., Vol. 120, No. 50, 1998 13137

A4 we obtain

This is the same equation that was obtained above (eq A10).

These conditions are applicable for our experiments used to

determine k_Tfor Cp(CO)₂(PMe₃)MoH at all temperatures

investigated, from -55 to -25 °C. For these experimental

conditions with Cp(CO)₂(PMe₃)MoH, k_TC< k_T[Ph₃C⁺]; note,

d[product]

) k_obs[Ph₃C⁺]

dt

(A8)

where

however, that this treatment is independent of k_CTand k_TC.

Acknowledgment. This research was carried out at Brook-

haven National Laboratory under contract DE-AC02-98CH10886

with the U.S. Department of Energy and was supported by its

Division of Chemical Sciences, Office of Basic Energy Sciences.

We thank Dr. Norman Sutin for help in the design and

interpretation of the cis/trans kinetics experiments. We thank

Dr. Carol Creutz, Dr. Robert Hembre, and Dr. Mark Andrews

for helpful discussions. We thank Dr. Soley S. Kristja´nsdo´ttir

for providing us with a copy of her program for NMR line-

broadening simulations, and Prof. Jack Norton for providing a

copy of KINPAR, a Macintosh program for determination of

activation parameters.

k_CTk

k_TC

k_T

k_obs

)

^T+ k_C[cis-MH] )

+ k_C[cis-MH] (A9)

(

)

(

)

K_eq

In our experiments, since k_T/K_eq. k_C, then

k

k_obs

)

^T[cis-MH] ) k_T[trans-MH]

(A10)

K_eq

In the more general case, k_obs) k_T[trans-MH] + k_C[cis-MH],

and whether k_Tor k_Cdominates depends on the relative

reactivities and concentrations of the two isomers.

The reaction mechanism in Scheme 2 was also modeled by

numerically integrating the rate equations (i.e., the steady-state

assumption was not used). This modeling showed that when

k_T[Ph₃C⁺]₀) k_TC, where [Ph₃C⁺]₀is the initial Ph₃C⁺concen-

tration, and [MH] . [Ph₃C⁺], the observed first-order rate

constant is only 16% greater than the one predicted by the

steady-state approximation {k_CTk_T/(k_TC+ k_T[Ph₃C⁺]₀)}[MH].

Under these conditions the growth of product appears first-order,

even though the change in the trans concentration is significant,

|d[trans-MH]/dt| ≈ k_CT[cis-MH] ≈ k_TC[trans-MH].

Supporting Information Available: Plots of kinetics data:

plot of absorbance vs time for the reaction of (CO)₅ReH with

Ph₃C⁺BF₄(Figure S1); plot of ln(A_t- A_∞) vs time for the

-

reaction of (CO)₅ReH with Ph₃C⁺BF₄^-(Figure S2); plot of k_obs

vs [MH] for (CO)₅ReH (Figure S3), Cp*(CO)₃WH (Figure S4),

Cp(CO)₃WH (Figure S5), cis-(CO)₄(PPh₃)MnH (Figure S6),

Cp(CO)₃MoH and Cp(CO)₃MoD (Figure S7), and trans-Cp-

(CO)₂(PCy₃)MoH and trans-Cp(CO)₂(PCy₃)MoD (Figure S8);

Eyring plot for cis-Cp(CO)₂(PCy₃)MoH f trans-Cp(CO)₂-

-

(PCy₃)MoH isomerization (Figure S9); and plot of log k_Hvs

3. For experiments using Cp(CO)₂(PMe₃)MoH, with [trans-

MH] . [Ph₃C⁺], the steady-state approximation is not needed.

pK_Rfor Ph_n(p-MeOC₆H₄)_3-nC⁺BF₄^-, for n ) 0, 1, 2, 3 (Figure

+

S10) (6 pages, print/PDF). See any current masthead page for

ordering information and Web access instructions.

d[product]

) k_obs[Ph₃C⁺], where k_obs) k_T[trans-MH]

dt

JA9820036

Article Doi

Hydride transfer reactions of transition metal hydrides: Kinetic hydricity of metal carbonyl hydrides

DOI: 10.1021/ja9820036

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