Alkyne ligand enhancement of the substitution lability of mononuclear osmium, ruthenium, and iron carbonyls

Article abstract of DOI:10.1021/ja9640049

The kinetic influence of an alkyne ligand, hexafluorobut-2-yne (HFB), as been investigated by studying the reactions of phosphines (PR₃) with the complexes M(CO)₄(η²-HFB) (M = Fe, Ru, Os). The rate of production of M(CO)₃(PR₃)(η²-HFB) is independent of the nature and concentration of the phosphine in all cases, indicating that the rate-controlling step is CO dissociation. The kinetic parameters, k₁ (s^-1, 25 °C). ΔH* (kJ mol^{- 1}), and ΔS* (cal mol^-1 K^-1) are: 9.5, 88.2 ± 2.3, 70 ± 10 (Fe), 1.25 x 10^-2, 103.6 ± 2.4, 66 ± 8.6 (Ru); 3.5 x 10^-3, 99.5 ± 0.8, 21 + 2.7 (Os). When the rate constants at 25 °C for M(CO)₄(η²-HFB) are compared to those of the parent M(CO)₅, the ratios are ~3 x 10¹³, 1.8 x 10² and 1 x 10⁷ for M = Fe, Ru, an Os, respectively. Clearly the alkyne increases the substitution lability, and the effect is spectacular with Fe, very large with Os, and substantial but relatively more modest with Ru. The increased lability results mainly from a reduced ΔH* of ~80, 10, and 33 kJ mol^-1 for Fe, Ru, and Os, respectively, and this is attributed largely to stabilization of the transition state by 4-electron donation from the alkyne ligand. Also reported are kinetics of formation of some trans M(CO)₂(PR₃)₂(η²-HFB) complexes and an extension of earlier work on the Os(CO)₅/PPh₃ system.

Full text of DOI:10.1021/ja9640049

1434
J. Am. Chem. Soc. 1998, 120, 1434-1440
Alkyne Ligand Enhancement of the Substitution Lability of
Mononuclear Osmium, Ruthenium, and Iron Carbonyls
Jean Pearson, Jason Cooke, Josef Takats, and R. B. Jordan*
Contribution from the Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta, Canada T6G 2G2
ReceiVed NoVember 18, 1996
Abstract: The kinetic influence of an alkyne ligand, hexafluorobut-2-yne (HFB), has been investigated by
studying the reactions of phosphines (PR3) with the complexes M(CO)4(η2-HFB) (M ) Fe, Ru, Os). The rate
of production of M(CO)3(PR3)(η2-HFB) is independent of the nature and concentration of the phosphine in all
cases, indicating that the rate-controlling step is CO dissociation. The kinetic parameters, k1 (s-1, 25 °C),
∆H* (kJ mol-1), and ∆S* (cal mol-1 K-1) are: 9.5, 88.2 ( 2.3, 70 ( 10 (Fe); 1.25 × 10-2, 103.6 ( 2.4, 66
( 8.6 (Ru); 3.5 × 10-3, 99.5 ( 0.8, 21 ( 2.7 (Os). When the rate constants at 25 °C for M(CO)4(η2-HFB)
are compared to those of the parent M(CO)5, the ratios are ∼3 × 1013, 1.8 × 102 and 1 × 107 for M ) Fe,
Ru, and Os, respectively. Clearly the alkyne increases the substitution lability, and the effect is spectacular
with Fe, very large with Os, and substantial but relatively more modest with Ru. The increased lability results
mainly from a reduced ∆H* of ∼80, 10, and 33 kJ mol-1 for Fe, Ru, and Os, respectively, and this is attributed
largely to stabilization of the transition state by 4-electron donation from the alkyne ligand. Also reported are
kinetics of formation of some trans M(CO)2(PR3)2(η2-HFB) complexes and an extension of earlier work on
the Os(CO)5/PPh3 system.
Introduction
to generate labile 17- or 19-electron species.8 Although useful,
these processes are by no means universal in their application.
In addition to external reagents, spectator ligands on metal
carbonyl derivatives also can faciltate CO substitution through
steric and electronic effects. Well-known examples of this
phenomenon are the trans-effect9 and cis-labilization.10,11
Recent reports from our laboratories12 on the synthesis and
reactivity of M(CO)4(η2-RCtCR) (M ) Ru, Os) complexes
indicate that these are remarkably reactive species. For example,
with M ) Os and R ) H, CO exchange is complete in 1 h at
0-10 °C. These compounds also appear to act as electrophiles
in unusual reactions with other 18-electron metal carbonyls to
give a variety of dimetallacyclic products.
Transition-metal carbonyls are one of the most important
classes of organometallic complexes.1 They serve as useful
starting materials for other organometallic complexes,2 as
stoichiometric reagents in numerous organic transformations,
and as catalyst precursors for important catalytic processes.3
Because the transition-metal carbonyls normally are saturated
18-electron species, they tend to be kinetically inert and a
challenge in the field has been to discover new methods to bring
about CO substitution.
Several reagents are known to promote removal of CO
ligands. For example, Me3NO can oxidize CO to CO2 and the
latter is readily lost and replaced by other ligands.4 Other
promoters of CO elimination that have been used include
R3PO,5 KOMe,6 KH, and NaBH4.7 Another approach is to use
electron-transfer reagents, including supported transition metals,
The present kinetic study was undertaken to elucidate the
mechanism of substitution on M(CO)4(η2-HFB) (M ) Fe, Ru,
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Organomet. Chem. 1969, 17, 309.
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S0002-7863(96)04004-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/05/1998

Alkyne Ligand Enhancement in Fe, Ru, and Os Carbonyls
J. Am. Chem. Soc., Vol. 120, No. 7, 1998 1435
Scheme 1
Os; HFB ) hexafluorobut-2-yne) complexes and to quantify
the labilizing effect of the alkyne ligand by comparison to their
M(CO)5 parents. The results provide further data for compari-
sons of periodic trends in reactivity for metal carbonyl systems.
It is known that the order of reactivity for the M(CO)5 sys-
tems is Ru . Os > Fe. It is shown here that the alkyne
ligand dramatically changes this order to Fe . Ru > Os and
the Fe(CO)4(η2-alkyne) system undergoes a spectacular 13
orders of magnitude increase in substitution rate compared to
Fe(CO)5.
Figure 1. Absorbance-time variation for the reaction of 0.10 M PMe3
with Os(CO)4(η2-HFB) at 14.6 °C in dichloromethane. Numbers indicate
the monitoring wavenumber; circles are experimental points, and curves
are least-squares fits to a biphasic model.
Results
The general reaction system is shown in Scheme 1. The
formation of the mono- and bisphosphine complexes has been
observed primarily by infrared spectroscopy. For many of the
phosphines studied, both k1 and k2 could be determined, but
with some, k2 was so large relative to k1 that only the latter
could be measured. The kinetics with various phosphines has
been explored mainly with M ) Os.
Reactions with Os(CO)4(η2-HFB). Synthetic studies13 have
allowed the characterization of some representative intermediate
and final products of phosphine substitution in this system. The
13C{1H} NMR shows that the mono- and disubstituted phos-
phine derivatives correspond to the axial and diaxial, trans
isomers as shown in I and II, respectively.
Figure 2. Absorbance-time variation for the reaction of 0.10 M
P(OPh)3 with Os(CO)4(η2-HFB) at 20.0 °C in dichloromethane.
Numbers indicate the monitoring wavenumber; circles are experimental
points, and curves are least-squares fits to a biphasic model.
as an electronic effect parameter for phosphine ligands. It is
noteworthy that the carbonyl stretching frequencies for the
Os(CO)4-n(PR3)n(η2-HFB) species give a good linear correlation
with δ. The νCO values decrease with increasing δ as expected
if a larger δ reflects greater σ-donor strength of PR3 and
therefore greater π back-bonding to CO.
If two or more equivalents of PMe3 or PPh3 are used, the
product isolated is Os(CO)2(PR3)2(η2-HFB) (R ) Me, Ph), and
the 19F and 13C{1H} NMR spectra show that this species is the
trans phosphine isomer II. This observation also demonstrates
that compounds II are resistant to further substitution by
phosphine ligands.
For this system, a wide range of phosphines have been studied
and in many cases both substitution steps can be kinetically
characterized. The variation of k2 with phosphine illustrates
some aspects of the influence of phosphines as a spectator
ligands.
The kinetics of the reactions have been monitored by infrared
spectroscopy, using both reactant and product bands to deter-
mine the rate constants. The reactant Os(CO)4(η2-HFB) is
characterized by strong bands at 2070 and 2028 cm-1, while
the Os(CO)3(PR3)(η2-HFB) derivatives have bands at about
2083-2097, 2008-2013, and 1980-1998 cm-1. The final
products, Os(CO)2(PR3)2(η2-HFB), have characteristic bands in
the 1984-2034 and 1912-1970 regions. Details for each
compound are given in the Supporting Information. Bodner
et al.14 determined the 13C chemical shift parameter (δ,
ppm) for a series of Ni(CO)3(PR3) compounds and suggested δ
The reaction may be monitored in several complementary
ways. The disappearance of reactant monitored at 2070 or 2028
cm-1 allows k1 to be determined with little interference from
the second step. The growth and decay of I can be observed at
∼2010 or 2090 cm-1, while II can be observed in the 1930 or
2000 cm-1 regions. Examples of absorbance-time curves are
shown in Figure 1 for the reaction with PMe3. Least-squares
fits of this type of data to a biphasic model allowed values of
the first-order rate constants k1 and k2 to be determined. The
conformance to a first-order rate law indicates that the buildup
of CO to ∼ 4 mM during the reaction causes no inhibition of
the rate. For some systems, k2 could not be evaluated because
it is so large, relative to k1, that the first step is entirely rate
controlling. An example of this behavior is shown for triphenyl
phosphite in Figure 2. The small absorbance at 1999 cm-1
indicates that very little I is present, and the half-time for
disappearance of reactant (2070 cm-1) and formation of product
II (1970 cm-1) are essentially identical, as shown by the dashed
lines on Figure 2.
In principle, the rate constants k1 and k2 are pseudo-first-
order because the phosphine or phosphite concentration (>0.04
M) was always larger than that of Os(CO)4(η2-HFB) (<1 ×
(13) Mao, T. Ph.D. Thesis, University of Alberta, 1996.
(14) Bodner, G. M.; May, M. P.; McKinney, L. E. Inorg. Chem. 1980,
19, 1951.

1436 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
Pearson et al.
Table 2. Kinetic Results for Reaction of PPh3 with Os(CO)5 in
Table 1. Some PR3 Properties and Rate Constants for Reactions
with Os(CO)4(η2-HFB) (k1) and Os(CO)3(PR3)(η2-HFB) (k2)a
Decalin
cone
[PPh3], M
temp, °C
105k1, s-1
105k2, s-1
ref
temp,
°C
δc, angle,d E′R (ER),e 104k1, 104k2,
0.15
0.15
0.15
0.15
0.019-0.112
0.15
0.15
115.4
110.0
109.6
100.0
96.0
90.2
90.0
11.9
6.10
6.42
1.96
1.23
0.60
0.54
0.138
this work
this work
this work
this work
Basolo et al.
this work
this work
Basolo et al.
Basolo et al.
PR3
PMe3
PMe2Ph
PMePh2
PPh3
PPh3
PPh3
PPh3
pKab ppm deg kcal mol-1 s-1
s-1
14.5
14.6
14.8
14.3
14.3
14.3
14.3
8.65 5.05 118
6.5 4.76 122
4.57 4.53 136
2.73 4.30 145
2.73 4.30 145
2.73 4.30 145
2.73 4.30 145
3.84 4.50 145
8.69 5.54 132
2.6 3.18 107
9.7 6.32 170
26 (39)
33 (44)
39 (57)
43 (75)
43 (75)
43 (75)
43 (75)
43 (75)
44 (61)
48 (52)
8.54 2.50
8.58 2.16
9.11 3.44
7.33 9.02
4.09f 10.7f
8.70g 10.6g
9.69h 7.05h
8.11 12.0
8.46 10.6
8.29 38.7
1.05a
0.662
0.586
0.33a
0.10
0.037-0.075
0.0335
86.0
76.4
P(p-tolyl)3 14.8
a Average of values reported at different [PPh3].
PEt3
P(OMe)3
PCy3
14.7
14.8
15.5
cm-1 were used to determine k1 and k2. Formation of the latter
product was indicated also by the decrease of absorbance at
1943 cm-1 for long reaction times. The infrared bands of the
phosphine derivatives are consistent with previous reports17 and
with axial phosphine ligands in an overall trigonal bipyramidal
structure.
The results for this system are summarized in Table 2 and
activation parameters are given in Table 3. The k1 is well
defined by the disappearance of Os(CO)5, and our values agree
with those of Basolo and co-workers15 as shown by the
temperature-dependence plot of data from both studies in Figure
3. Since it was not practical to follow the reaction to com-
pletion, k2 is less certain and was evaluated by holding k1 fixed
during the least-squares analysis of the data at 1943 and 1899
cm-1. The resulting k2’s do give a satisfactory temperature plot,
also shown in Figure 3.
67 (116) 8.97 fast
72 (65) 7.99 fast
P(OPh)3
14.3 -2.0 1.69 128
a
The solvent is dichloromethane unless otherwise indicated.
b Tolman, C. A. Chem. ReV. 1977, 77, 313. c The 13C NMR chemical
shift of Ni(CO)3(PR3) relative to Ni(CO)4 given in ref 14. d Golovin,
M. N.; Rahman, M. M.; Belmonte, J. E.; Giering, W. P. Organome-
tallics 1985, 4, 1981. e Ligand repulsive energies for CpRh(CO)(L) (ER′)
and Cr(CO)5(L) (ER) from: Choi, M.-G; Brown, T. L. Inorg. Chem.
1993, 32, 5603. Revised values for PMe2Ph and PMePh2 have been
provided by Professor Brown. f In THF. g In toluene. h In Decalin.
10-3 M). However, it was found that the rates for all of the
reactions are independent of the concentration of the phosphine
or phosphite. This feature was established for concentrations
of triphenylphosphine between 0.043 and 0.25 M in dichlo-
romethane (DCM) and typically was checked for other systems
between 0.1 and 0.2 M. Therefore, the rate law for both steps
is given by
Reactions with Ru(CO)4(η2-HFB). Synthetic studies13 have
shown that the final product in the presence of excess phosphine
is the Ru analogue of II. With PPh3, the disappearance of reac-
tant at 2074 and 2042 cm-1 and appearance of product at 2010
and 1949 cm-1 were monitored, and the PPh3 concentration was
varied from 0.10 to 0.25 M. For P(OPh)3 and PCy3 the 2042-
cm-1 peak could not be used because it is also present in the
rate ) ki[Os(CO)4(η2-HFB)]
(1)
where i ) 1 or 2. Furthermore, the value of k1 was found to
be independent of the nature of the phosphine or phosphite,
even though these had a wide range of basicities and steric
requirements. However, k2 does depend on the nature of the
phosphine or phosphite, but this is not unexpected because the
P-donor ligand, already present as a spectator ligand in I, will
affect its reactivity.
The solvent dependence of the reactions with PPh3 was
explored by studies in THF, toluene, and decalin, in addition
to the most often used solvent, dichloromethane (DCM).
The results for various phosphines are given in Table 1 and
activation parameters are summarized in Table 3. Full details
are given in the Supporting Information.
Reaction of Os(CO)5 with PPh3. This study was undertaken
to permit comparison of the reactivity of Os(CO)5 and Os(CO)4-
(η2-HFB). The reaction has been studied previously by Basolo
and co-workers,15 but it has been noted16 that the results appear
anomalous because the ∆S* is some 60 J mol-1 K-1 smaller
than for the Fe and Ru analogues. Since this system makes an
important point of comparison, it seemed worthwhile to study
it again. Basolo and co-workers used a batch method and
analyzed for disappearance of Os(CO)5 at 1993 cm-1 in Decalin,
and found that the rate was independent of the concentration
and nature of the phosphine. We have studied the reaction with
PPh3 in Decalin by continuous monitoring with the sample
heated in the same infrared cell used for the Os(CO)4(η2-HFB)
study. For the determination of the rate constants, the disap-
pearance of Os(CO)5 at 2034 and 1991 cm-1 and appearance
of Os(CO)4(PPh3) at 1943 cm-1 and Os(CO)3(PPh3)2 at 1899
product, and product formation was observed at 1984 cm-1
.
The kinetic behavior and rate law for Ru(CO)4(η2-HFB) are
the same as for Os(CO)4(η2-HFB) except that k2 was too large
to be evaluated for PPh3 as well as for P(OPh)3 and PCy3. The
results are summarized in Table 4.
Reactions with Fe(CO)4(η2-HFB). With the recent avail-
ability of this compound,18 it was of obvious interest to study
its phosphine substitution reactivity in order to complete the
triad comparison with the analogous Ru and Os compounds.
Preliminary studies indicated that the reaction of Fe(CO)4(η2-
HFB) with PPh3 is much faster than that of either Os(CO)4(η2-
HFB) or Ru(CO)4(η2-HFB) and is too fast to study by our
infrared system, even at -20 °C. Therefore, 19F NMR
monitoring has been used to study the reaction in the -30 to
-40 °C range with the disappearance of reactant being observed
at -55.8 ppm.
The first substitution with PPh3 gives the axial isomer with
structure I, and the second substitution results in the diaxial
isomer II, mirroring the Ru and Os behavior. However, with
Fe, the second substitution is not complete and an equilibrium
is established between I and II as the concentration of
dissociated CO builds up in the sealed NMR tube. An
unanticipated result with the bulky PCy3 ligand is that mono-
substitution step gives not only the axial isomer I but also the
equatorial isomer. Further reaction to give the disubstituted
(15) Shen, J.-K.; Gao, Y.-C.; Shi, Q.-Z.; Basolo, F. Inorg. Chem. 1989,
28, 4304.
(16) Huber, B. J.; Poe¨, A. J. Inorg. Chim. Acta 1994, 227, 215.
(17) Collmann, J. P.; Roper, W. R. J. Am. Chem. Soc. 1965, 87, 4008.
L′Eplattenier, F.; Calderazzo, F. Inorg. Chem. 1968, 7, 1290.
(18) Cooke, J.; Takats, J. J. Am. Chem. Soc. 1997, 119, 11088.

Alkyne Ligand Enhancement in Fe, Ru, and Os Carbonyls
J. Am. Chem. Soc., Vol. 120, No. 7, 1998 1437
Table 3. Kinetic Results for Substitution Reactions on Os(CO)4(L) (k1) and Os(CO)3(PR3)(L) (k2)
temp range,
∆H1*,
∆S1*,
∆H2*,
∆S2*,
PR3
PPh3
PPh3
P(OPh)3
PCy3
PPh3
(L)
solvent
°C
kJ mol-1
J mol-1 K-1
kJ mol-1
J mol-1 K-1
(η2-HFB)
(η2-HFB)
(η2-HFB)
(η2-HFB)
(CO)b
DCM
THF
DCM
DCM
DEC
3.0-23.6
3.0-23.6
3.0-23.6
10.6-24.1
90-115
99.5 ( 0.8
98.3 ( 1.3
96.1 ( 2.6
99.3 ( 1.0
133.4 ( 2.6
41.8 ( 2.7
32.7 ( 4.4
30.5 ( 8.9
41.2 ( 3.6
20.8 ( 7.0
99.4 ( 1.6
95.6 ( 1.9
43.2 ( 5.6
30.6 ( 6.5
168 ( 2.9c
92 ( 7.5c
a The solvents are: DCM, dichloromethane; THF, tetrahydrofuran; DEC, Decalin; ∆H* and ∆S* are given to an additional figure to avoid
round-off errors in recalculation, and error limits are one standard deviation. b See Table 2 for details. c k2 could only be evaluated between 100 and
115 °C.
Table 4. Kinetic Results for Reactions of PR3 with Ru(CO)4(L)
Compoundsa
k, s-1
∆H,*
∆S,*
PR3
(L)
6.5 °C
25 °C
kJ mol-1 J mol-1 K-1
PPh3 (η2-HFB)b 7.5 × 10-4
1.25 × 10-2 103.6 ( 2.4 66.0 ( 8.6
d
POPh3 (η2-HFB) 8.2 × 10-4 c (1.3 × 10-2
)
)
PCy3 (η2-HFB) 7.3 × 10-4 c (1.3 × 10-2
d
PPh3 COe
PPh3
73 × 10-6
115.6
63.6
72.4
56.9
59.0
68.2
76.6
f
3.2 × 10-6 125.9
f
PCy3
PMePh2
29 × 10-6
115.9
f
1.3 × 10-6 124.3
0.43 × 10-6 129.7
0.45 × 10-6 132.1
f
P(OEt)3
g
PnBu3 PnBu3
a The solvent is DCM unless otherwise indicated. b Temperature
range is 1.8-15.8 °C; errors are one standard deviation. c Experimental
rate constants at temperatures of 6.8 and 6.2 °C for P(OPh)3, and PCy3,
respectively. d Calculated assuming the ∆H* for PPh3. e From Huq, R.;
Poe¨, A. J.; Chawla, S. Inorg. Chim. Acta 1980, 38, 121, in cyclohexane
over a temperature range of 30-50 °C. f With various entering groups,
in Decalin (PPh3, PMePh2) and heptane (PCy3, P(OEt)3) as given by:
Chen, L.; Poe¨, A. J. Inorg. Chem. 1989, 28, 3641. g Poe¨, A.; Twigg,
M. V. Inorg. Chem. 1974, 13, 2982, in Decalin.
Figure 3. Temperature dependence of the reaction of PPh3 with Os-
(CO)5 (k1) and Os(CO)4(PPh3) (k2) in Decalin; (0, O), results from
this study; (9) results from Basolo and co-workers, ref 20.
compound with structure II is observed under synthetic condi-
tions when CO does not build up in the system, but this step is
not observed in the sealed NMR tube used for the kinetic studies.
Full details of the nature of these products will be the subject
of a forthcoming publication, and only the kinetics of the
disappearance of reactant in the first substitution step will be
considered here.
The general kinetic behavior for the k1 step is the same as
for the other systems. The rate of CO displacement is inde-
pendent of the concentration and nature of the phosphine (PPh3,
PCy3, and P(OPh)3). The first-order rate constants are sum-
marized in Table 5. The activation parameters, determined from
the temperature dependence of k1, are given in Table 6.
Table 5. Kinetic Results for Reactions of PR3 with
Fe(CO)4(η2-HFB) in CD2Cl2
a
PR3
temp, °C
[PR3], M
103k1, s-1 b,c
PPh3
-30.7
-33.5
-33.5
-33.5
-33.7
-33.7
-36.7
-39.9
0.20
0.30
0.30
0.30
0.20
0.10
0.20
0.20
2.16 ( 0.06 (2.19)
1.34 ( 0.05 (1.30)
1.22 ( 0.05 (1.30)
1.30 ( 0.03 (1.30)
1.28 ( 0.05 (1.25)
1.30 ( 0.05 (1.25)
0.694 ( 0.03 (0.704)
0.375 ( 0.07 (0.375)
PCy3
P(OPh)3
PPh3
PPh3
PPh3
PPh3
PPh3
a Rate constants determined by 19F NMR monitoring of the disap-
pearance of the singlet at δ -56.0 ppm in a solution initially containing
2.5 × 10-2 M Fe(CO)4(η2-HFB) and the indicated [PR3]. b Errors are
one standard deviation determined from least-squares fits of the
integrated intensity for 30 points covering ∼90% reaction. c Values in
brackets calculated from ∆H* ) 88.2 kJ mol-1 and ∆S* ) 69.56 J
Discussion
All of the systems studied here show rates that are indepen-
dent of the concentration and nature of the entering group and
are consistent with the usual dissociative mechanism shown in
Scheme 2. This is also indicated by the positive ∆S* values.
With a steady-state assumption for the concentration of the
dissociative intermediate, this mechanism predicts that the
observed rate constant (k1 or k2) should be given by
mol-1 K-1
.
substitution on Os(CO)5, and the results in Table 2 relate to
CO dissociation from this species.
As already noted, the new data for Os(CO)5 are in good
agreement with the earlier study,15 but the extended temperature
range gives revised ∆H* and ∆S* values that are higher by 6
kJ mol-1 and 22 J mol-1 K-1, respectively. The ∆S* now is
distinctly positive as expected for a dissociative mechanism,
but is still 44 J mol-1 K-1 lower than that for Ru(CO)5. For
the dissociation of methylacrylate from the M(CO)4(η2-meth-
ylacrylate) systems16 and of CO from M(CO)4(η2-HFB), dis-
cussed below, the ∆S* is 12 and 24 J mol-1 K-1 smaller for
Os than Ru, and this may be a general feature of dissociation
from these M(CO)4L systems.
ki ) kidkis[PR3]/k-id[CO] + kis[PR3]
(2)
and under our experimental conditions of [PR3] . [CO], the
observations indicate that kis[PR3] . k-id[CO] so that eq 2
reduces to eq 3.
ki ) kid
(3)
Therefore, the results in Tables 1, 3, 4, and 5 give the kinetic
parameters related to kid for CO dissociation from Os(CO)4(η2-
HFB), Ru(CO)4(η2-HFB), and Fe(CO)4(η2-HFB), respectively.
With obvious modifications, Scheme 2 also can apply to
It is qualitatively apparent that the alkyne HFB has a dramatic
effect on the CO lability. Substitution on Fe(CO)4(HFB) was
measured at -30 to -40 °C while substitution on Fe(CO)5 is

1438 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
Pearson et al.
Table 6. Comparison of Reactivity of M(CO)5 and M(CO)4(L)
Systems with PPh3
geometries, and M-C and C-O bond lengths, so that there is
nothing in the ground state to indicate why ∆H* is 20 kJ mol-1
smaller for substitution on Ru(CO)5. In Fe(CO)5,21c the M-C
bond is ∼0.15 Å shorter (compared to ∼0.1 Å expected from
covalent radii), and the C-O bond is ∼0.02 Å longer than in
the Ru and Os analogues. These differences are consistent
with stronger π-back-bonding in Fe(CO)5, and may account, at
least in part, for the estimated 35-50 kJ mol-1 higher ∆H* for
Fe(CO)5.
k1, s-1
25 °C
∆H*,a
∆S*,a
metal species
kJ mol-1
J mol-1 K-1
Fe(CO)4(η2-HFB)b
9.5
88 ( 2.3
70 ( 10
(75)
Fe(CO)5
∼3 × 10-13
5.4 × 10-15
1.25 × 10-2
7.0 × 10-5
3.2 × 10-6
3.5 × 10-3
3.3 × 10-10
1.3 × 10-12
(167)
c
d
Fe(CO)4PPh3
176 ( 4.7
104 ( 2.4
114.5 ( 3.9
126 ( 1.4
99.5 ( 0.8
133 ( 2.6
168 ( 2.9
73 ( 11
66 ( 8.6
60 ( 12
72 ( 4.4
42 ( 2.7
21 ( 7.0
92 ( 7.5
Ru(CO)4(η2-HFB)b
e
Ru(CO)5
f
Ru(CO)4PPh3
Os(CO)4(η2-HFB)b
Two theoretical studies have given CO dissociation ener-
gies for Fe(CO)5, Ru(CO)5, and Os(CO)5 of 191, 138, and
b,g
Os(CO)5
b,g
145 kJ mol-1 22 or 194, 129, and 177 kJ mol-1 23
, . The Fe sys-
Os(CO)4PPh3
a Values determined from least-squares analysis of published data;
errors are one standard deviation. b This work, in dichloromethane
unless otherwise indicated. c Estimated in ref 19. d Siefert, E. E.;
Angelici, R. J. J. Organomet. Chem. 1967, 8, 374, in Decalin. e Huq,
R.; Poe¨, A. J.; Chawla, S. Inorg. Chim. Acta 1980, 38, 121, in
cyclohexane. f Johnson, B. F. G.; Lewis, J.; Twigg, M. V. J. Chem.
Soc., Dalton Trans. 1975, 1876, in heptane. g In Decalin.
tem is complicated by the fact that the lowest energy form of
{Fe(CO)4} is a triplet,24 calculated22, 23 to be ∼7.5 kJ mol-1
below the singlet, but CO dissociation is assumed to proceed
by the spin-allowed path to the singlet. Pulsed laser pyrolysis25
has given a first bond dissociation energy of 172 kJ mol-1 for
Fe(CO)5 in modest agreement with the calculations. More
recent calculations26 give lower values of 156, 101, and 120 kJ
mol-1 for Fe(CO)5, Ru(CO)5, and Os(CO)5, respectively. For
Ru(CO)5 and Os(CO)5, these calculated gas-phase dissociation
energies are ∼15 kJ mol-1 lower than our solution phase ∆H*
values, but the predicted difference in ∆H* of 19 kJ mol-1 is
in excellent agreement with the 19 kJ mol-1 found here. If
one simply relies on the calculated energy differences, then
Fe(CO)5 is predicted to have ∆H* ≈ 170 kJ mol-1, consistent
with the 167 kJ mol-1 estimated by Basolo and co-workers.15
It should be noted that the ∆S* is always more positive for
the Ru than for the Os systems (see Tables 1 and 4), and this
makes ∆G* more favorable for Ru(CO)5 by ∼10 kJ mol-1 at
25 °C. This may reflect some difference in transition state
structure as suggested originally by Basolo and co-workers.15
Indeed, the latest calculations26 indicate an earlier transition state
with a significantly shorter M-C bond for Os than for Ru.
The ground-state structures of Os(CO)4(η2-HFB) and Ru-
(CO)4(η2-HFB) are very similar.19 The alkyne C-C bond length
is 1.276 Å and the -CF3 is bent back 38° from the C-C axis
in both. These features show the normal trend from CtC
toward CdC bonding for η2-alkyne coordination. The M-CO
bond lengths are the same, within 0.01 Å of those in the parent
M(CO)5 compounds, so that there is no indication that the alkyne
has any special effect on the M-CO bonding that would explain
the high reactivity of the alkyne derivatives relative to the
pentacarbonyls of Ru and Os.
Scheme 2
too slow to measure at accessible temperatures; Fe(CO)4PPh3
was studied at 160 to 180 °C. Similarly but less dramatically,
the Os(CO)4(η2-HFB) system was studied in the 10 °C range,
while Os(CO)5 was studied from 76 to 115 °C.
The rate constants and activation parameters for PPh3
substitution on M(CO)5, M(CO)4PPh3, and M(CO)4(η2-HFB)
systems are summarized in Table 6. A comparison of the rate
constants at 25 °C for M(CO)5 and M(CO)4(η2-HFB) shows
that the latter is more reactive by factors of ∼3 × 1013, 1.8 ×
102, and 1 × 107 for M ) Fe, Ru, and Os, respectively. The
rate acceleration for iron is spectacular. The effect of the HFB
resides largely in a lowering of the ∆H* by ∼80, 12, and 33 kJ
mol-1 relative to M(CO)5, with the interesting result that, while
Fe(CO)5 is the least reactive pentacarbonyl, Fe(CO)4(η2-HFB)
is the most reactive alkyne tetracarbonyl derivative.
Increased reactivity due to changing the ligands or the metal
may be attributed to either or both of ground-state destabilization
or transition-state stabilization. Specifically, the alkyne might
destabilize the M(CO)4(η2-alkyne) complex relative to M(CO)5
by 4-electron repulsion between the filled metal d orbitals
and the alkyne π orbital.19 But the alkyne might stabilize the
{M(CO)3(alkyne)} transition state relative to {M(CO)4} to make
the alkyne complex more reactive. This is akin to the well-
documented cis-effect in Mn(CO)5X systems, where the theo-
retical analysis of Lichtenberger and Brown10a suggests that
stabilization of the intermediate by π-donor ligands is important
for cis-labilization. Darensbourg and co-workers11 have ob-
served examples of the cis-effect for η1-O2CCH3,-11a F,-11b and
1,2-substituted benzene ligands.11c Ideas concerning π-stabiliza-
tion of unsaturation and filled-filled repulsions have been
reviewed and greatly expanded recently by Caulton.20
The structure of Fe(CO)4(η2-HFB) has yet to be determined,
but all the spectroscopic information indicates that it has the
same ligand arrangement as the other two. The infrared spectra
in the CO stretching region (in pentane) are similar with peaks
at 2124, 2058, and 2032 cm-1 (Fe), 2143, 2073, and 2042 cm-1
(Ru), and 2149, 2069, 2061, and 2029 cm-1 (Os).
(19) Marinelli, G.; Streib, W. E.; Huffman, J. C.; Caulton, K. G.; Gagne´,
M. R.; Takats, J. Dartiguenave, M.; Chardon, C.; Jackson, S. A.; Eisenstein,
O. Polyhedron, 1990, 9, 1867.
(20) Caulton, K. G., New J. Chem. 1994, 18, 25.
(21) (a) Huang, J.; Hedberg, K.; Pomeroy, R. K. Inorg. Chem. 1990,
29, 3923. (b) Huang, J.; Hedberg, K.; Pomeroy, R. K. Organometallics
1988, 7, 2049. (c) Lu¨thi, H. P.; Siegbahn, P. E. M.; Almlo¨f, J. J. Phys.
Chem. 1985, 89, 2156. (d) The solid-state structure of Fe(CO)5 has been
determined by: Braga, D.; Greponi, F.; Orpen, A. G. Organometallics 1993,
12, 1481.
(22) Li, J.; Schreckenbach, G.; Zeigler, T. J. Am. Chem. Soc. 1995, 117,
486.
(23) Ehlers, A. W.; Frenking, G. Organometallics 1995, 14, 423.
(24) Poliakoff, M.; Weitz, E. Acc. Chem. Res. 1987, 20, 408. Poliakoff,
M.; Turner, J. J. J. Chem. Soc., Dalton Trans. 1974, 2276.
(25) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc. 1984,
106, 3905.
Ground-state effects are easiest to deal with because spec-
troscopic and structural information are available. In the gas
21a,b
phase, Ru(CO)5 and Os(CO)5
have essentially identical
(26) Klobukowski, M.; Decker, S. Details to be published.

Alkyne Ligand Enhancement in Fe, Ru, and Os Carbonyls
J. Am. Chem. Soc., Vol. 120, No. 7, 1998 1439
by PR3 might increase k2d due to the greater steric bulk of the
spectator phosphine, but M-C back-bonding might increase
because PR3 is a poorer π-acid and better σ-donor than CO,
and this should decrease k2d. It must be noted that there is no
compelling evidence for the latter effect from the structures of
Fe(CO)3(PR3)2 and Fe(CO)4(PR3) complexes.30 Since phos-
phines are better σ-donors than CO, they might be expected to
reduce the stabilizing effect of π-donation from the alkyne in
the transition state. But steric interactions between phosphine
and alkyne might be relieved in the transition state as the alkyne
moves to donate π-electrons to the site trans to the phosphine
that has been vacated by the leaving group.
The k2 results are summarized in Table 1. It is noteworthy
that the k2 values vary from ∼3 times smaller to ∼4 times larger
than k1. Thus replacement of CO by phosphine may either
reduce or increase the dissociation rate of the second CO. It
appears that basicity of the phosphine, as measured by the pKa
or δ values in Table 1, is not a major factor because the most
acidic (P(OPh)3) and most basic (PCy3) phosphines have k2
values too large to determine. There is a tendency for a larger
cone angle to give a larger k2, but PEt3 and especially P(OMe)3
and P(OPh)3 are more reactive than expected on this basis.
However, there is some uncertainty about the cone angles,31
especially for PEt3 and P(OMe)3, and a judicious choice of
values could remove some of this discrepancy. There is a
reasonable correlation of k2 with the steric repulsion parameter
E′R of Choi and Brown,32 as can be seen by inspection of Table
1, where the data are presented in the order of the E′R values.
Figure 4. Possible stabilization of the incipient dissociative intermedi-
ate by π-electron donation from an alkyne ligand.
The simplest explanation of the labilizing effect of the alkyne
seems to be in terms of transition state stabilization.19 This
can occur through π-electron donation from the CtC system
into the vacant site in the dissociative intermediate as illustrated
in Figure 4.
In effect, the alkyne acts as a 4-electron donor ligand in the
transition state, while π-back-bonding to the alkyne may be
largely retained. There is some indication of 4-electron donation
by an alkyne from the structure of the coordinatively unsaturated
Os(η2-C2Ph2)(CO)(PiPr3)2 complex that has a roughly square
pyramidal ligand arrangement with the C-C bond lengthened
to 1.32 Å, and a back bending angle of 46°.27 The latter
parameters show a greater tendency to CdC bonding than in
the saturated M(CO)4(η2-HFB) systems described above. In a
similar system, Caulton and co-workers28 have suggested that
4-electron donation by PhCtCPh may account for the ease of
phosphine dissociation from Ru(η2-C2Ph2)(CO)2(PtBu2Me)2. The
theoretical work26 on M(CO)4(η2-C2H2) systems indicates that
the acetylene moves in the transition state as indicated in Figure
4 to a position with an angle of ∼120° to the remaining apical
CO, and the C-C bond lengthens to 1.32 Å.29
The question remains why this transition state stabilization
seems to be in the order Fe . Os > Ru. It is expected that the
extent of stabilization should be related to the energy difference
between the filled alkyne π orbital and the vacated metal dσ
orbital in the transition state. A qualitative explanation for the
greater substitution lability of Fe(CO)4(η2-HFB) may lie with
the lower energy of the 3d orbitals and the well-known tendency
of first-row transition-metal complexes to favor 18-electron
configurations compared to the second- and third-row metal
complexes. Nevertheless, the huge magnitude of the accelera-
tion for Fe(CO)4(η2-HFB) and the more modest effect for
Ru(CO)4(η2-HFB) seem to require further explanation. The
ongoing calculations26 predict the correct relative reactivity order
of Fe > Os > Ru for the M(CO)4(η2-C2H2) systems, but a
detailed rationalization would be premature at this time.
The factors affecting k2 (i.e., k2d) also are difficult to assess,
probably because of compensating effects. Replacement of CO
There appears to be a steric plateau for E′R < ∼40 kcal mol-1
.
The data for E′R g 39 give a correlation coefficient of 0.98 for
log (k2) versus E′R and correctly predicts that PCy3 and P(OPh)3
should give much higher reactivity than any of the other
phosphines studied. Although one might expect a similar
correlation with ER,33 the P(OPh)3, P(OMe)3, and PEt3 systems
do not correlate well.
The most closely analogous studies for k2 are those of Chen
and Poe¨34 on Ru(CO)4L systems; some of these results are given
in Table 4. The reactivity changes were attributed primarily to
steric effects. It is notable that, in the above study, all the P
donors give a lower reactivity than the Ru(CO)5 parent complex,
but k2 is larger than k1 for some of the Os(CO)3(L)(η2-HFB)
systems. This could reflect the combined steric effects of the
PR3 and HFB ligands.
Conclusions
The potential for alkyne ligands to act as 2-electron or
4-electron donors enables them to support a variety of structural
motifs and to undergo diverse reactions. Previous work from
our laboratories12 and the present study clearly demonstrate a
new facet of alkyne ligands, their ability to greatly enhance the
rate of CO substitution in d8 M(CO)4(η2-HFB) (M ) Fe, Ru,
Os) complexes. The increased lability is not unexpected in view
of the possibile π-donor ability of 2-electron donor alkynes,
but the magnitude of the effect is truly remarkable, especially
in the case of Fe(CO)4(η2-HFB). If, as we argue, the enhanced
substitution lability is due to stabilization of the dissociative
(27) Espuelas, J.; Esteruelas, M. A.; Lahoz, F. J.; Lo´pez, A. M.; Oro, L.
A.; Valero, C. J. Organomet. Chem. 1994, 468, 223.
(28) Ogasawara, M.; MacGregor, S. A.; Streib, W. E.; Folting, K.;
Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1996, 118, 10189.
(29) A reviewer suggested that the electron-withdrawing ability of
hexafluorobutyne might promote lability, given the fomal M(II) oxidation
state in the metallacyclopropene extreme of the M-(HFB) interaction (ref
19). The high CO stretching frequencies in M(CO)4(η2-HFB) were sited as
supporting evidence for oxidative addition that would give less M-CO
back-bonding. In response, we note that Haszeldine and co-workers (J.
Chem. Soc. A 1970, 1964) characterized several analogues of this proposal,
namely, derivatives of (OC)4Fe(II) with fluorinated ligands such as the
ferracyclopentane, Fe(CO)4(η1,η1-(CF2)4), and Fe(CO)4(η2-(C2F4)) which
contains the ferracyclopropane extreme of iron-olefin bonding. The former
compound has somewhat higher CO stretching frequencies (2150, 2092,
2072, 2056 cm-1) than Fe(CO)4(η2-HFB), but CO substitution by PPh3 is
sluggish even at 80 °C, giving 53% of the monophosphine derivative, but
leaving 24% unreacted starting material after 24 h. The latter compound
also does not exhibit CO lability. These observations further emphasize
the special labilizing effect of the alkyne in M(CO)4(η2-alkyne) compounds,
especially in comparison with alkene-type ligands.
(30) Glaser, R.; Yoo, Y.-H.; Chen, G. S.; Barnes, C. L. Organometallics
1994, 13, 2578 and references therein. Li, C.; Stevens, E. D.; Nolan, S. P.
Organometallics 1995, 14, 3791.
(31) White, D.; Coville, N. J. AdV. Organomet. Chem. 1994, 36, 95.
White, D.; Taverner, B. C.; Coville, N. J.; Wade, P. W. J. Organomet.
Chem. 1995, 495, 41.
(32) Choi, M.-G.; Brown, T. L. Inorg. Chem. 1993, 32, 5603.
(33) Choi, M.-G.; Brown, T. L. Inorg. Chem. 1993, 32, 1548 and
references therein.
(34) Chen, L.; Poe¨, A. J. Inorg. Chem. 1989, 28, 3641.

1440 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
Pearson et al.
The temperature typically was constant to (0.1 °C. The infrared cell
(Graseby Specac) was a gastight model with CaF2 windows and
amalgamated foil spacer and gaskets and a stainless steel body.
The infrared spectrometer was a Bomem MB 100 controlled by the
software package Spectra-Calc running on an IBM-386 type computer.
Special software was developed to control the timing and display the
spectra during the kinetic run. Each spectrum was normalized to the
absorbance of the initial spectrum in a spectral region where reactants
and products did not absorb. The data file from each run was converted
to a “text” format so that it could be further analyzed by least-squares
methods to determine the rate constants. This analysis was done on
an IBM-486 type computer using a least-squares program based on
the Marquardt algorithm and written in BASIC.
The NMR samples for Fe(CO)4(η2-HFB) study were prepared by
adding 225 µL of 0.050 M Fe(CO)4(η2-HFB) in CD2Cl2 and 225 µL
of phosphine in CD2Cl2, both at -78 °C under N2, to a thick-walled
NMR tube also at -78 °C. The contents of the NMR tube was
subjected to three freeze-pump-thaw cycles, and then the tube was
flame sealed and kept at -78 °C until the spectometer probe was
equilibrated at the required temperature. The 19F NMR spectra were
run on a Bruker WH-200 instrument and externally referenced to CFCl3.
The temperature was monitored by an external Sensotek BAT-10
copper-constantan thermocouple immersed in toluene in a 5-mm OD
NMR tube. The probe temperature was steady for 10 min before the
sample was introduced. The temperature after the run was measured
and the recorded temperature is the average of the beginning and final
temperatures. In general, the temperature drift was <0.1 °C. The
integrated intensities were obtained from standard routines, using
absolute intensities and a common set of processing parameters for
each series of experiments. The time dependence of the intensity was
fitted to a first-order rate law with the same least-squares program used
to fit the infrared data.
16-electron intermediate by π-donation from the π orbital of
the alkyne, then the nature of the alkyne should affect the rate
of CO substitution. This issue will be addressed in a future
publication on the reactions of Os(CO)4(η2-HCCH) with various
phosphines.
Experimental Section
Materials. The hexafluorobut-2-yne (HFB) derivatives Os(CO)4-
(η2-HFB) and Ru(CO)4(η2-HFB) were prepared as described previ-
ously,12 as was Os(CO)5.35 The phosphines were used as obtained from
Aldrich and were stored and handled under a nitrogen atmosphere.
Solvents were purified by distillation from appropriate drying agents:
sodium benzophenone ketyl for toluene and tetrahydrofuran; phosphorus
pentoxide or CaH2 for dichloromethane; sodium metal for Decalin. The
NMR spectra were recorded in dry, deoxygenated CD2Cl2 unless
otherwise indicated.
The synthesis and characterization of Fe(CO)4(η2-HFB)18 and some
iron,18 and ruthenium and osmium13 products, such as M(CO)3(PR3)-
(η2-HFB) and M(CO)2(PR3)2(η2-HFB) (M ) Fe, Ru, Os; R ) Me, Ph),
will be reported in detail elsewhere.
Kinetic Measurements. Stock solutions of Os(CO)4(η2-HFB) or
Ru(CO)4(η2-HFB) were prepared in the required solvent under nitrogen
in serum-capped vials. An appropriate volume (2-5 mL) of the stock
solution was transferred by syringe to a vial inside a container in a
nitrogen atmosphere in a bath slightly below the ultimate reaction
temperature. Solid phosphine was added to the vial before the alkyne
complex; liquid phosphine was added by syringe to the solution of
alkyne complex. A portion of the mixed solution was transferred by
syringe to the infrared cell and this was placed in the thermostating
system and the spectrum of the sample was recorded at appropriate
intervals. Before each run the cell was brought to temperature in the
spectrometer and a solvent blank was recorded.
The thermostating system consisted of a liquid circulating bath filled
with 2-propanol (Haake Q bath) or silicone oil (Lauda K6 bath for
temperatures > 30°). Liquid circulated from the bath through a double-
walled container with an inside diameter just large enough to accom-
modate the infrared cell. The temperature of the cell was continuously
monitored by a Doric copper-constantan digital thermometer that had
been calibrated against a Hewlett-Packard Pt resistance thermometer.
Acknowledgment. The authors thank the Natural Sciences
and Engineering Research Council of Canada for financial
support for this work under the Cooperative Grants Program,
and the Killam Trust for a research allowance to J. C.
Supporting Information Available: Infrared spectra of
products and full kinetic results (4 pages). See any current
masthead page for ordering and Web access instructions.
(35) (a) Rushman, P.; van Buuren, G. N.; Shiralian, M.; Pomeroy, R. K.
Organometallics 1983, 2, 693. (b) Washington, J.; McDonald, R.; Takats,
J.; Menashe, N.; Reshef, D.; Shvo, Y. Organometallics 1995, 14, 3996.
JA9640049