Mechanisms of reactions of dihydrogen complexes: Formation of trans-[RuH(H2)(dppe)2]+ and substitution of coordinated dihydrogen

Article abstract of DOI:10.1021/ic9902267

The reactions between cis-[RuH₂(DPPE)₂] and a number of acids in THF solution (DPPE = Ph₂PCH₂CH₂PPh₂) show biphasic kinetics, with initial formation of trans-[RuH(H₂)(DPPE)₂]⁺ followed by slower substitution of coordinated dihydrogen by the anion of the acid. The formation of the dihydrogen complex is a second-order process that occurs with an inverse kinetic isotope effect and rate constants k_HX strongly dependent on the nature of the acid. There is a linear correlation between the values of log k_HX for cis-[RuH₂(DPPE)₂] and the related cis-[FeH₂(PP₃)] [PP₃ = P(CH₂CH₂PPh₂)₃] that leads to two parameters, S and R, that can be used as a measure of the selectivity and intrinsic reactivity of the dihydride toward acids. The possible contributions to the values of these parameters are discussed, especially the role of the isomerization of the starting complex and the basicity of the reacting species. The substitution of coordinated dihydrogen in trans-[RuH(H₂)(DPPE)₂]⁺ occurs through a simple dissociative mechanism instead of the more complicated one previously proposed for substitutions in the analogous Fe complex; the mechanistic change is associated with the relative strength of the M-H₂ and M-P(chelate) bonds.

Full text of DOI:10.1021/ic9902267

Inorg. Chem. 1999, 38, 5067-5071
5067
Mechanisms of Reactions of Dihydrogen Complexes: Formation of trans-[RuH(H2)(dppe)2]+
and Substitution of Coordinated Dihydrogen
Manuel G. Basallote,* Joaqu´ın Dura´n, M. Jesu´s Ferna´ndez-Trujillo, and M. Angeles Ma´n˜ez
Departamento de Ciencia de los Materiales e Ingenier´ıa Metalu´rgica y Qu´ımica Inorga´nica,
Facultad de Ciencias, Universidad de Ca´diz, Apartado 40, Puerto Real, 11510 Ca´diz, Spain
ReceiVed February 24, 1999
The reactions between cis-[RuH2(DPPE)2] and a number of acids in THF solution (DPPE ) Ph2PCH2CH2PPh2)
show biphasic kinetics, with initial formation of trans-[RuH(H2)(DPPE)2]+ followed by slower substitution of
coordinated dihydrogen by the anion of the acid. The formation of the dihydrogen complex is a second-order
process that occurs with an inverse kinetic isotope effect and rate constants kHX strongly dependent on the nature
of the acid. There is a linear correlation between the values of log kHX for cis-[RuH2(DPPE)2] and the related
cis-[FeH2(PP3)] [PP3 ) P(CH2CH2PPh2)3] that leads to two parameters, S and R, that can be used as a measure
of the selectivity and intrinsic reactivity of the dihydride toward acids. The possible contributions to the values
of these parameters are discussed, especially the role of the isomerization of the starting complex and the basicity
of the reacting species. The substitution of coordinated dihydrogen in trans-[RuH(H2)(DPPE)2]+ occurs through
a simple dissociative mechanism instead of the more complicated one previously proposed for substitutions in
the analogous Fe complex; the mechanistic change is associated with the relative strength of the M-H2 and
M-P(chelate) bonds.
Introduction
systematic kinetic study of reactions involving dihydrogen
complexes, and the first results showed that the reaction
mechanisms are not always in agreement with the behavior
anticipated from simple considerations.18-20 For example, kinetic
data for protonation of [FeH2(PP3)] [PP3 ) P(CH2CH2PPh2)3]
with several acids to form [FeH(H2)(PP3)]+ are consistent with
a mechanism in which the dihydrogen complex is formed
through a series of dihydrogen-bonded structures resulting from
attack of the coordinated hydride by the acid molecule HX.18
Despite the need of a cis-trans isomerization to give trans-
[FeH(H2)(DPPE)2]+ (DPPE ) Ph2PCH2CH2PPh2), the proto-
nation of cis-[FeH2(DPPE)2] is faster, which was considered as
evidence of the initial attack by the acid to the cis-dihydride
followed by rapid isomerization.19 On the other hand, substitu-
tion of coordinated H2 in trans-[FeH(H2)(DPPE)2]+ also shows
interesting kinetic features which suggest that these reactions
do not occur through a simple mechanism involving exclusively
the coordination site occupied by the leaving ligand.20 On the
contrary, substitutions in cis-[RuCl(nitrile)(DPPE)2]+ complexes
occur through a simple dissociative mechanism with formation
of [RuCl(DPPE)2]+ as intermediate.21 In the present paper we
report a kinetic study of reactions involving the well-
Dihydrogen complexes have been the subjects of extensive
work during the past years, and their chemistry has been
reviewed with special emphasis on the synthetic and structural
aspects as well as on their role in catalytic processes and their
differences with classical dihydrides.1,2 Despite the large amount
of information available, the mechanistic aspects of reactions
of dihydrogen complexes are far from being well understood,
and discussions are often based on qualitative observations,
theoretical considerations, and the conclusions of a limited
number of kinetic works. Thus, the weakness of the metal-
dihydrogen bond3-6 and the existence of related coordinatively
unsaturated complexes7-9 indicate a dissociative behavior in
substitution reactions, but there are few reports on the kinetic
details of these reactions.1,10-17 We have started recently a
(1) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.
(2) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913.
(3) Hay, P. J. J. Am. Chem. Soc. 1987, 109, 705.
(4) Maseras, F.; Dura´n, M.; Lledo´s, A.; Bertra´n, J. Inorg. Chem. 1989,
28, 2984.
(5) Pacchioni, G. J. Am. Chem. Soc. 1990, 112, 80.
(6) Maseras, F.; Dura´n, M.; Lledo´s, A.; Bertra´n, J. J. Am. Chem. Soc.
1991, 113, 2879.
(7) Aresta, M.; Giannocaro, P.; Rossi, M.; Sacco, A. Inorg. Chim. Acta
1971, 5, 115.
(8) Bianchini, C.; Laschi, F.; Peruzzini, M.; Ottaviani, F. M.; Vacca, A.;
Zanello, P. Inorg. Chem. 1990, 29, 3394.
(9) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R.
H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem.
Soc. 1996, 118, 5396.
(10) Gonza´lez, A. A.; Zhang, K.; Hoff, C. D. Inorg. Chem. 1989, 28, 4295.
(11) Millar, J. M.; Kastrup, R. V.; Melchior, M. T.; Horvath, I. V.; Hoff,
C. D.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 9643.
(12) Halpern, J.; Cai, L.; Desrosiers, P. J.; Lin, Z. J. Chem. Soc., Dalton
Trans. 1991, 717.
(15) Zhang, K.; Gonza´lez, A. A.; Hoff, C. D. J. Am. Chem. Soc. 1989,
111, 3627.
(16) Hauger, B. E.; Gusev, D.; Caulton, K. G. J. Am. Chem. Soc. 1994,
116, 208.
(17) Bakhmutov, V. I.; Bertra´n, J.; Esteruelas, M. A.; Lledo´s, A.; Maseras,
F.; Modrego, J.; Oro, L. A.; Sola, E. Chem. Eur. J. 1996, 2, 815.
(18) Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Ma´n˜ez, M. A.;
Rodr´ıguez de la Torre, J. J. Chem. Soc., Dalton Trans. 1998, 745.
(19) Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Ma´n˜ez, M. A.
J. Chem. Soc., Dalton Trans. 1998, 2205.
(13) Oglieve, K. E.; Henderson, R. A. J. Chem. Soc., Chem. Commun. 1992,
441.
(14) Gusev, D.; Kuhlman, R.; Renkema, K. B.; Eisenstein, O.; Caulton,
K. G. Inorg. Chem. 1996, 35, 6775.
(20) Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Gonza´lez, G.;
Ma´n˜ez, M. A.; Mart´ınez, M. Inorg. Chem. 1998, 37, 1623.
(21) Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Ma´n˜ez, M. A.
J. Chem. Soc., Dalton Trans. 1998, 3227.
10.1021/ic9902267 CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/14/1999

5068 Inorganic Chemistry, Vol. 38, No. 22, 1999
Basallote et al.
characterized22-28 trans-[RuH(H2)(DPPE)2]+ complex, whose
results provide new information about the mechanism of both
the formation of dihydrogen complexes and the substitution of
coordinated H2.
cis-[RuH2(DPPE)2] + HX f trans-[RuH(H2)(DPPE)2]+ +
X- (3)
(ca. 5 min), but experiments at lower temperatures demonstrate
that substitution is substantially slower than protonation. The
product of reaction with trifluoroacetic acid has not been
reported previously, but it can be reasonably identified as trans-
[RuH(CF3COO)(DPPE)2]+ from the observation of a singlet
Results
The dihydrogen complex trans-[RuH(H2)(DPPE)2]+ can be
prepared by addition of 1 equiv of HBF4‚Et2O to cis-[RuH2-
(DPPE)2] in ether under H2 atmosphere (eq 1).23 We carried
2
(65.2 ppm) and a quintet (-15.8 ppm, JH,P ) 19 Hz) in the
phosphorus and proton spectra, respectively. It was not isolated
because it is formed in equilibrium with the same intermediate
described above for the reaction with HBF4, which in a few
minutes converts partially to an uncharacterized cis compound,
probably cis-[Ru(η2-CF3COO)(DPPE)2]+, with two triplets in
cis-[RuH2(DPPE)2] + HBF4 f
trans-[RuH(H2)(DPPE)2]+ + BF4 (1)
-
out NMR experiments at -60 °C in acetone-d6 solutions and
confirmed that reaction with a large excess of HBF4 leads to
complete conversion to trans-[RuH(H2)(DPPE)2]+ without
evidence of any reaction intermediate or side product. However,
if the reaction is carried out at 25 °C, the product consists of a
mixture of the dihydrogen complex and another species
characterized by a singlet at 57.8 ppm and a quintet at -3.72
ppm (2JH,P ) 21 Hz) in the phosphorus and proton NMR spectra,
respectively. These signals disappear when the NMR samples
are cooled below -40 °C, and the mixture of both complexes
converts quantitatively to trans-[RuH(MeCN)(DPPE)2]+ upon
addition of an excess of acetonitrile.
Despite [RuH2(DPPE)2] being known to exist as a mixture
of the cis and trans isomers, with the cis form being the major
species,23,29 there is no formation of detectable amounts of trans-
[RuH2(DPPE)2] during the conversion of the cis-dihydride to
the trans-hydride-dihydrogen complex. From published NMR
data, the equilibrium constant for eq 2 is close to 0.05 in both
2
the 31P spectrum (60.0 and 58.6 ppm, JP,P ) 18 Hz).
Fortunately, the time scale for this conversion is slower than
that corresponding to eqs 3 and 4, and it does not interfere with
the kinetic study discussed below.
It has been previously shown30 that protonation of trans-
[RuH(Cl)(DPPE)2] leads to a chlorodihydrogen complex, and
our NMR experiments with HCl excess confirmed the sequence
of reactions 5-7, the slowest one being formation of trans-
trans-[RuH(H2)(DPPE)2]+ + X-
f
trans-[RuHX(DPPE)2] + H2 (4)
[RuCl(H2)(DPPE)2]+. This complex does not undergo further
reaction with HCl, and instead of the expected substitution of
H2, it forms trans-[RuH(MeCN)(DPPE)2]+ upon addition of
MeCN excess, in a way similar to that previously observed for
the reaction with CO.30
cis-[RuH2(DPPE)2] a trans-[RuH2(DPPE)2] Ki (2)
cis-[RuH2(DPPE)2] + HCl f trans-[RuH(H2)(DPPE)2]+ +
C6D6 and THF solutions,23,29 but the absence of the trans isomer
during our experiments indicates a lower value in acetone
solution, in agreement with previous observations by Morris
and co-workers,23 who were also unable to detect the trans
isomer in acetone-d6 at room temperature. We derived Ki values
from the intensity of the phosphorus NMR signals and found
that they are strongly dependent on the nature of the solvent,
ranging from 0.05 (C6D6) to 0.01(CD2Cl2) and even lower
(nonmeasurable by NMR) in THF and acetone solutions.
The formation of trans-[RuH(H2)(DPPE)2]+ as the initial
product of reaction between cis-[RuH2(DPPE)2] and an excess
of other acids (HCl, HBr, or CF3COOH) was also confirmed
using low-temperature NMR, although in those cases the process
is followed by substitution of H2 by the anion of the acid. At
room temperature, the sequence of reactions in eqs 3 and 4 is
completed within the time required to obtain the first spectrum
Cl- (5)
trans-[RuH(H2)(DPPE)2]+ + Cl-
trans-[RuH(Cl)(DPPE)2] + H2 (6)
f
trans-[RuH(Cl)(DPPE)2] + HCl f
trans-[RuCl(H2)(DPPE)2]+ + Cl- (7)
Kinetics of Reaction of cis-[RuH2(DPPE)2] with Acids. We
initially tried to study the kinetics of protonation of cis-[RuH2-
(DPPE)2] using the electrochemical procedure previously de-
scribed,18 but the reaction is too fast and occurs at 25 °C within
the time required to mix the reagents (ca. 1 s). Preliminary
stopped-flow experiments using a diode-array detector showed
that the reactions with HCl and CF3COOH occur with biphasic
kinetics under pseudo-first-order conditions (acid excess, THF
solution, 25.0 °C). The time scale of the first step is on the
order of a few milliseconds, whereas the second step lasts for
almost 1 s. Accurate values of the first rate constant were
obtained from kinetic traces at 300 nm, which show a rapid
absorbance increase that can be fitted by a single exponential.
The values of the second rate constant were then obtained from
the fit of traces at 330 nm by two consecutive exponentials,
keeping fixed the value of the first one. Detailed listings of both
rate constants are given in Table S1 (Supporting Information).
Although for reactions showing biphasic kinetics there is always
(22) Morris, R. H.; Sawyer, J. F.; Shiralian, M.; Zubkowski, J. D. J. Am.
Chem. Soc. 1985, 107, 5581.
(23) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.;
Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991,
113, 4876.
(24) Bautista, M. T.; Earl, K. A.; Morris, R. H.; Sella, A. J. Am. Chem.
Soc. 1987, 109, 3780.
(25) Ashworth, T. W.; Singleton, E. J. Chem. Soc., Chem. Commun. 1976,
705.
(26) Cappellani, E. P.; Maltby, P. A.; Morris, R. H.; Schweitzer, C. T.;
Steele, M. R. Inorg. Chem. 1989, 28, 4437.
(27) Saburi, M.; Aoyagi, K.; Takahashi, T.; Uchida, Y. Chem. Lett. 1990,
601.
(28) Albeniz, A.; Heinekey, D. M.; Crabtree, R. H. Inorg. Chem. 1991,
30, 3632.
(29) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R.
H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375.
(30) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.; D’Agostino,
C. Inorg. Chem. 1994, 33, 6278.

Reactions of Dihydrogen Complexes
Inorganic Chemistry, Vol. 38, No. 22, 1999 5069
Table 2. Effect of the Incoming Ligand and the Solvent on the
Activation Parameters for the Substitution of Coordinated H2 in
trans-[RuH(H2)(dppe)2]+
incoming ligand
MeCN
solvent
∆Hq/(kJ mol-1
)
∆Sq/J K-1 mol-1
)
THF
89 ( 1
92 ( 1
85 ( 4
92 ( 5
91 ( 3
91 ( 5
73 ( 4
88 ( 4
65 ( 13
85 ( 18
80 ( 12
78 ( 17
acetone
MeCN
THF
THF
THF
CF3COO-
Cl-
Br-
The activation parameters for H2 substitution were determined
using several entering ligands and solvents (Tables 2 and S3).
The values in THF for the reaction with MeCN, Cl-, Br-, and
CF3COO- agree within error, showing that the rate-limiting step
is the same in all cases, which suggests a limiting dissociative
mechanism. The changes of the activation parameters with the
solvent nature are also consistent with a D mechanism, the lower
activation barrier in acetonitrile solution being a consequence
of the preferential solvation of the coordinatively unsaturated
transition state.
The results of kinetic studies with DCl and CF3COOD are
included in Tables S1 and 1; in both cases the protonation is
faster than for the undeuterated acids and there is an inverse
kinetic isotope effect (kie; see values in Table 1). On the
contrary, there is a very small positive kie of ca. 1.1 associated
with the substitution step (k2). Although the observation of a
positive kie is in agreement with those previously reported for
substitution of coordinated dihydrogen in other complexes,15-17
the value of 1.1 is not a measure of the isotope effect associated
with substitution of H2 in trans-[RuH(H2)(DPPE)2]+ because
protonation with DX leads to a fluxional complex in which only
one of the three hydrogens is deuterated.
It has been pointed out above that reaction of cis-[RuH2-
(DPPE)2] with HCl excess does not end with the formation of
trans-[RuHCl(DPPE)2]; the complex is further protonated to
trans-[RuCl(H2)(DPPE)2]+, although in a very slow process that
does not affect the determination of the k1obs and k2obs values.
It would be very interesting to obtain kinetic data for the
formation of the chlorodihydrogen complex and the subsequent
elimination of HCl, so we prepared trans-[RuHCl(DPPE)2] and
attempted kinetic studies. Unfortunately, although low-temper-
ature NMR experiments confirmed that protonation of this
complex is slow compared to the reactions of cis-[RuH2-
(DPPE)2], we have been unable to obtain accurate kinetic data
for these reactions.
Figure 1. Plots of k1obs versus acid concentration for the reaction of
cis-[RuH2(DPPE)2] with HBF4‚Et2O (a), CF3COOH (b), and HCl (c).
Some points at higher concentrations of HBF4‚Et2O have been omitted
for clarity.
Table 1. Second-Order Rate Constants and Kinetic Isotope Effects
for the Reaction of cis-[RuH2((DPPE)2] with Acids in THF Solution
at 25.0 °C
(kHX
theor
/
a
HX
kHX/(M-1 s-1
)
kDX/(M-1 s-1
)
kHX/kDX kDX
)
HBF4‚Et2O (1.12 ( 0.08) × 103
CF3COOH (9.2 ( 0.6) × 104 (11.5 ( 0.4) × 104 0.80 ( 0.06 0.87
HCl
(1.7 ( 0.2) × 106
(4.5 ( 0.7) × 106 0.38 ( 0.07 0.47
a Theoretical values calculated by the procedure given in ref 18.
a problem for determining the order of occurrence of both steps,
the fit of the diode-array data only gives reasonable spectra for
the reaction intermediate assuming that the rapid step occurs
first, in agreement with the NMR observation described above.
Thus, the values of k1obs in Table S1 correspond to the formation
of trans-[RuH(H2)(DPPE)2]+, whereas k2obs corresponds to the
slower substitution of coordinated H2. Data in Table S1 show
that k2obs remains unaffected when the nature or the concentra-
tion of acid is changed, whereas the values of k1obs are
significantly different for every acid and show a linear depen-
dence with the acid concentration (eq 8, Figure 1). The values
of the second-order rate constant kHX are given in Table 1.
k1obs ) kHX[HX]
(8)
The protonation of cis-[RuH2(DPPE)2] with HBr is so rapid
that it occurs within the mixing time of the stopped-flow
instrument and kinetic traces only show the substitution step
(Table S1). On the contrary, protonation with HBF4‚Et2O is very
slow, and a single exponential can fit the traces at 300 and 330
nm, yielding similar values for the observed rate constant. The
values so derived are also given in Table S1, and they are
assigned to k1obs because substitution is now faster than
protonation and the whole process becomes controlled by the
formation of the dihydrogen complex. This conclusion was
confirmed by additional experiments of protonation with HBF4‚
Et2O and added MeCN; the protonation step is accelerated, and
the curves reveal the slower substitution of H2 by MeCN with
a rate constant similar to k2obs for the reaction with other acids
(Table S2, Supporting Information). The values of k1obs increase
with the concentration of added acetonitrile, surely because the
increased dielectric constant favors dissociation of the acid and
leads to faster protonation of cis-[RuH2(DPPE)2] through direct
proton attack.
Discussion
Mechanism of Formation of Dihydrogen Complexes. The
major kinetic features for the formation of trans-[RuH(H2)-
(DPPE)2]+ are similar to those previously observed for trans-
[FeH(H2)(DPPE)2]+ and cis-[FeH(H2)(PP3)]+,18,19 i.e., second-
order kinetics with an inverse kie and large changes in the rate
constant with the nature of the acid. The reactivity of cis-[RuH2-
(DPPE)2] with the different acids increases as HBF4 < CF3-
COOH < HCl < HBr, an ordering that does not parallel the
acid strength and was interpreted assuming that (H+, X-) ion
pairs react slower than HX molecules.18 We have also shown
previously19 a correlation between the rate constants for the
formation of the iron dihydrogen complexes with PP3 and DPPE
that can now be extended to include trans-[RuH(H2)(DPPE)2]+.
As the Fe-PP3 complex does not isomerize upon protonation,
we chose it as a reference, and the correlation is then represented
by eq 9, where S and R are measures of the selectivity and the
intrinsic reactivity of the Ru complex with acids, respectively.

5070 Inorganic Chemistry, Vol. 38, No. 22, 1999
Basallote et al.
log kHX(Ru-DPPE) ) S log kHX(Fe-PP ) + R
(9)
3
The values of kHX in Table 1 and ref 18 lead to S ) 1.66 and
R ) 2.87 (correlation coefficient 0.98; see Figure 2), and
extrapolation of the regression line predicts a value of log kHBr
higher than 7, in agreement with the experimental observation
that reaction of cis-[RuH2(DPPE)2] with HBr is so rapid that
kHBr could not be determined.
The use of the R and S parameters allows an easy ordering
of the complexes, which represents an important step for
determining the factors that cause the differences in the kinetics
of protonation. Thus, the intrinsic reactivity increases as (R
values in parentheses) cis-[FeH2(PP3)] (0.00) < cis-[FeH2-
(DPPE)2] (1.75) < cis-[RuH2(DPPE)2] (2.87) i.e., the ruthenium
complex is more reactive than the iron analogues, and the
reactivity of both DPPE complexes is higher than that of the
PP3 complex despite the need of isomerization to trans-[MH-
(H2)(DPPE)2]+. We previously19 considered this as evidence of
the initial protonation of the cis-dihydride followed by rapid
isomerization to the trans product; in that case, the R values of
the dihydrides would simply reflect the different electronic
environments of coordinated hydrides in the PP3 and DPPE
complexes. However, Morris and co-workers have reported that
deprotonation of trans-[RuH(H2)(DPPE)2]+ with BuLi leads
initially to trans-[RuH2(DPPE)2], which converts to the cis
isomer in a slower process,23,29 and microscopic reversibility
indicates that isomerization must precede the protonation of the
Ru complex in the reverse reaction. Moreover, it has been
demonstrated31 that formation of the closely related DMPE
complex occurs preferentially through protonation of the trans-
dihydride despite the presence of a larger concentration of the
cis isomer (DMPE ) Me2PCH2CH2PMe2). Thus, although the
experimental observations for protonation with HX and depro-
tonation by BuLi cannot be directly compared, the possibility
that both processes occur through the formation of trans-[RuH2-
(DPPE)2] must be seriously considered. In that case, as the trans-
dihydride does not accumulate during the formation of the
dihydrogen complex, the reaction must occur according to the
mechanism in eqs 10 and 11, where the first step is an
Figure 2. Correlation between the log kHX values for cis-[FeH2(PP3)]
and the complexes cis-[RuH2(DPPE)2] (a) and cis-[FeH2(DPPE)2] (b).
Also included is the line for the reference compound cis-[FeH2(PP3)]
(c).
are two axial H-M-P groups, and the strong σ-donation from
the hydrides can be more easily delocalized through back-
donation from the metal to the phosphine through the common
d orbital. This remarkable difference of reactivity between
isomeric pairs of Ru complexes has been previously recognized
for substitution reactions.32-34 The higher basicity of trans-
dihydrides is also supported by the observation that the pKa
values of trans-[RuX(H2)(diphosphine)2]+ complexes are strongly
dependent on the nature of X, covering a range of 18 units with
a minimum acidity for X ) H.35
The correlation in eq 9 indicates19 that kHX values follow a
Bronsted-type relationship similar to that found in classical
proton-transfer reactions36 and proton transfers from metal
hydrides to bases.37-39 In those cases there is a linear correlation
between log k and the difference between the pKa values for
both acid-base pairs. By analogy, the values of kHX for reaction
with a common acid are expected to increase with the basicity
of the metal hydrides, which can be measured by the pKa of
the dihydrogen complex. As the pKa values for trans-[FeH(H2)-
(DPPE)2]+ and trans-[RuH(H2)(DPPE)2]+ in the scale defined
by Morris are 12 and 14.1, respectively,1,29 this would explain
the higher intrinsic reactivity of the Ru complex. However,
although the pKa of cis-[FeH(H2)(PP3)]+ is not available, it must
be40 higher than 12, and the intrinsic reactivity of the corre-
sponding dihydride should be higher than that found experi-
mentally, placing [FeH2(PP3)] higher in the R scale. The cause
of this apparent inversion of the ordering of R values is probably
a different meaning of the reported pKa values; whereas for the
PP3 complex it would have the usual meaning, the pKa of the
cis-[RuH2(DPPE)2] a trans-[RuH2(DPPE)]
trans-[RuH2(DPPE)2] + HX f
Ki (10)
trans-[RuH(H2)(DPPE)2]+ + X-
kHX(trans) (11)
unfavorable equilibrium (Ki , 1) and kHX(trans) is the rate
constant for HX attack to the trans-dihydride. The rate law for
this mechanism is similar to the experimental one with kHX
)
KikHX(trans); i.e., the values of kHX include the equilibrium
constant for cis-trans isomerization of the starting dihydride.
As Ki is independent of the acid used, the effect of isomerization
is included in the R parameter and consists of increasing it by
a constant value of log Ki. The value of Ki is very small for
cis-[RuH2(DPPE)2] (less than 0.01), so the contribution of
isomerization to R is -2.0 or even more negative, which leads
to a value of R larger than ca. 5 for trans-[RuH2(DPPE)2]. This
represents a reactivity higher than that of both its cis isomer
and cis-[FeH2(PP3)], which does not undergo isomerization. If
the mechanism is similar for the case of cis-[FeH2(DPPE)2], it
would indicate again an increased reactivity of the trans-
dihydride, probably because of a higher electron density on the
coordinated hydrides; for the case of cis-[MH2(dppe)2] there
(32) Isied, S. S.; Taube, H. Inorg. Chem. 1976, 15, 3070.
(33) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19,
1404.
(34) Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1982, 21, 1037.
(35) Rocchini, E.; Mezzetti, A.; Ru¨eger, H.; Burckhardt, U.; Gramlich, V.;
Del Zotto, A.; Martuzzini, P.; Rigo, P. Inorg. Chem. 1997, 36, 711.
(36) Bell, R. P. The Proton in Chemistry; Cornell University Press: Ithaca,
NY, 1973.
(37) Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1986,
108, 2257.
(38) Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1987,
109, 3945.
(39) Kristja´nsdo´ttir, S. S.; Loendorf, A. J.; Norton, J. R. Inorg. Chem. 1991,
30, 4470.
(31) Field, L. D.; Hambley, T. W.; Yau, B. C. K. Inorg. Chem. 1994, 33,
(40) Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Ma´n˜ez, M. A.
Unpublished results.
2009.

Reactions of Dihydrogen Complexes
Inorganic Chemistry, Vol. 38, No. 22, 1999 5071
DPPE complexes also includes the isomerization to the cis-
dihydride. The experimental values (pKa) and the values in the
absence of isomerization (pKa′) are related29 by eq 12, and pKa′
trans-[FeH(H2)(DPPE)2]+, whose kinetic data are not consistent
with a simple mechanism involving exclusively the coordination
site of the leaving ligand.20 On the contrary, a simple D
mechanism also operates in substitutions of cis-[RuCl(RCN)-
(DPPE)2]+,21 which suggests that H2 does not behave as a
leaving ligand in a way very different from that of other
monodentate ligands; it is the metal center which plays a
fundamental role in determining the substitution mechanism,
with a D mechanism being operative for the Ru complexes and
a chelate ring-opening mechanism for the iron analogue. The
reason for this change in the mechanism must be related to the
relative energy of the M-P(chelate) and M-H2 (or M-NCR)
bonds. The affinity of Fe(II) for the phosphine ligands is not
very high43 and favors the ring-opening mechanism in the iron
complexes, but the energy of the M-H2 bond decreases1,23,29
as Fe > Ru and favors the simple D mechanism for the
ruthenium complexes. The general behavior is probably more
complicated because the strength of both the M-P and M-H2
bonds also depends on the other ligands in the complex and
subtle modifications in the coordination environment can also
lead to a change in the substitution mechanism.
pKa ) pKa′ + log Ki
(12)
is higher than pKa by 1-2 units in the Ru complex and by an
unknown amount in the Fe complex. If the formation of the
dihydrogen complexes occurs through reaction of trans-[MH2-
(DPPE)2] (eqs 10 and 11), the basicity of the reacting species
is determined by the values of pKa′, so the intrinsic reactivity
of these complexes is higher than that of cis-[FeH2(PP3)].
Whereas the value of R for protonation of the dihydride
complexes appears to be reasonably explained by considering
both the isomerization to a more reactive species and the basicity
of the species attacked by the acid, the factors leading to a
different selectivity toward acids are not so clear. The selectivity
follows the order (S values in parentheses)
cis-[FeH2(DPPE)2] (0.50) < cis-[FeH2(PP3)] (1.00) <
cis-[RuH2(DPPE)2] (1.66)
and steric factors cannot be invoked because the Ru-DPPE
complex is 3 times more selective than its Fe analogue despite
the lower steric requirements. More work is in progress to
determine the factors that lead to a different selectivity.
Mechanism of Substitution of Coordinated Dihydrogen.
The kinetic data for substitution of dihydrogen in trans-[RuH-
(H2)(DPPE)2]+ strongly suggest a simple D mechanism. Actu-
ally, the NMR experiments demonstrate that, in the absence of
suitable entering ligands, the complex dissociates H2 and exists
in equilibrium with another species, probably in a way similar
to that observed for the DMPE analogue.31 The signal at 57.8
ppm in the phosphorus spectra had been previously observed
after argon was bubbled through a THF solution of the
dihydrogen complex and was tentatively assigned to the
coordinatively unsaturated species [RuH(DPPE)2]+.30 However,
related five-coordinate ruthenium(II) hydrides usually give rise
to resonances close to -22 ppm,31,41 and the observation of the
hydride signal at -3.72 ppm in our experiments suggests some
interaction with the solvent, especially in the NMR experiments
(acetone solution). The tendency of [RuH(DPPE)2]+ to form
trans-[RuH(solvent)(DPPE)2]+ has been previously recog-
nized.23,25,30
The activation enthalpy for dissociative loss of H2 in trans-
[RuH(H2)(DPPE)2]+ is 90 kJ/mol, close to the values found for
nitrile substitution in the related cis-[RuCl(nitrile)(DPPE)2]+
complexes, which also undergo substitution through a dissocia-
tive mechanism.21 Although it is tempting to use that value as
an estimation of the Ru-H2 binding energy, there can be
significant differences because of the possibility of energy
changes associated with solvation and/or structural reorganiza-
tion of the intermediate, as demonstrated for [RuCl(DPPE)2]+.21
Actually, although the NMR spectra of the five-coordinate
hydride do not show any evidence of agostic interactions or
structural rearrangement, the complex exchanges the hydride
with the hydrogen atom of a CH group in a way similar to that
of some analogues with related diphosphines, which form an
agostic interaction with a CH bond to compensate electron
deficiency.42
Experimental Section
The complex cis-[RuH2(DPPE)2] was prepared by the literature
procedure.23 The reagents RuCl3, CF3COOH, CF3COOD, HBF4‚Et2O,
CH3OD, chorotrimethylsilane, and bromotrimethylsilane were obtained
from Aldrich. Tetrahydrofuran, acetonitrile, acetone, acetone-d6, and
methanol were obtained from SDS (Solvants Documentation Syntheses).
The acids HCl and HBr were generated in THF solution from methanol
and choro- or bromotrimethylsilane, respectively. The deuterated acids
were obtained by the same procedure using CH3OD. All the synthetic,
NMR, and kinetic work was carried out under an inert atmosphere of
Ar or N2 with solvents dried by distillation from the appropriate drying
agents and deoxygenated by bubbling inert gas immediately before use.
The NMR experiments were carried out with a Varian Unity 400
spectrometer. The samples were prepared under an inert atmosphere,
and the acids were added after the samples were cooled at -90 °C
inside the NMR probe. The experiments were started by warming the
samples to the desired temperature before starting the acquisition of
the spectra.
The kinetic experiments were carried out with an Applied Photo-
physics SX17MV stopped-flow spectrophotometer. The solutions of
the metal complex and the acids were prepared using Schlenk techniques
and transferred under argon atmosphere to the instrument syringes using
Teflon tubes. The concentrations of acid solutions were determined
immediately before use by diluting with water (50 mL) an aliquot (1-3
mL) of the THF solution and titrating with KOH using phenolphthalein
indicator. The analysis of kinetic traces was carried out using the
standard software of the stopped-flow instrument, and the activation
parameters were derived from conventional Eyring plots of kinetic data
at different temperatures.
Acknowledgment. Financial support from Junta de Andalu-
c´ıa (Grant to Grupo FQM-0137) and the EU (Grant FEDER
96-0044) is gratefully acknowledged.
Supporting Information Available: Tables containing observed
rate constants for the reaction of cis-[RuH2(DPPE)2] with several acids
at 25 °C, for the reaction with HBF4‚Et2O in the presence of different
concentrations of added acetonitrile, and for the substitution of
coordinated dihydrogen at different temperatures. This material is
available free of charge via the Internet at http://pubs.acs.org.
The limiting D mechanism for substitutions in trans-[RuH-
IC9902267
(H2)(DPPE)2]+ differs from the behavior of the iron analogue
(41) Winter, R. F.; Hornung, F. M. Inorg. Chem. 1997, 36, 6197.
(42) Ogasawara, M.; Saburi, M. Organometallics 1994, 13, 1911.
(43) Barclay, J. E.; Leigh, G. J.; Houston, A.; Silver, J. J. Chem. Soc.,
Dalton Trans. 1988, 2865.