Synthesis, X-ray crystal structure, NMR characterization and theoretical calculations on [Cp2Ta(η2-H2)(CO)]+, the first thermally stable group 5 dihydrogen complex

Article abstract of DOI:10.1039/b002073g

Protonation of Cp₂TaH(CO) (Cp = C₅H₅, 1a; C₅H₄Bu^t, 1b) by HBF₄·Et₂O at -78°C in CH₂Cl₂ affords [CP₂TaH₂(CO)]BF₄ (2, 3) as mixtures of 2 isomers. The minor ones (2a, 2b) contain the known trans-dihydride [CP₂TaH₂(CO)]⁺ cations whereas the major ones (3a, 3b) are [CP₂Ta(η²-H₂)(CO)]BF₄, the first group 5 dihydrogen complexes. The crystal structure of the analogous complex 3a· BArf₄ recorded at 120 K confirms the presence of the coordinated dihydrogen ligand, which displays an H-H separation of 1.09(2) A in agreement with distances calculated from NMR data. Protonation of Cp₂TaH₂(SiMe₂Ph) by (Et₂O)₂ · HBArf₄ does not lead to an analogous silane derivative but to the new dinuclear complex [(CP₂TaH₂)₂(μ-H)](BArf₄). Variable temperature NMR studies were carried out on the dihydrogen complex [Cp₂Ta(H₂)(CO)]⁺ (3) and its isotopomers. The high field signal of [CP₂Ta(HD)(CO)]⁺ (3-d) shows a decoalescence at 208 K in both ¹H and ²D NMR, which allows us to calculate the barrier to rotation of HD (9.6 kcal mol^-1). The absence of decoalescence in the signal of 3 down to 173 K and the absence of a large kinetic isotope effect for the classical rotation of H₂ were demonstrated. These results are understood in terms of the presence of very large exchange couplings in a non-rotating dihydrogen ligand. The large barrier of rotation for the dihydrogen ligand in 3 was shown by DFT calculations to arise from a transition state in which the dihydrogen ligand is only coordinated through σ-donation from the H-H bond. The analogous phosphite and phosphine complexes {CP₂TaH₂[P(OMe)₃]}⁺ (4) and [Cp₂TaH₂(PMe₂Ph)]⁺ (5) were shown to be cis dihydrides, in agreement with DFT calculations on a model compound, to display exchange couplings in NMR and no isotope effect for the classical exchange of the hydride ligands.

Full text of DOI:10.1039/b002073g

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Synthesis, X-ray crystal structure, NMR characterization and
theoretical calculations on [Cp Ta(g2-H )(CO)]‘, the Ðrst
2
2
thermally stable group 5 dihydrogen complex
Sylviane Sabo-Etienne,a Venancio Rodriguez,a Bruno Donnadieu,a Bruno Chaudret,*a
Hassna Abou el Makarim,¤b Jean-Claude Barthelat,b Stefan Ulrich,c Hans-Heinrich Limbachc
and Claude Mo•sed
a L aboratoire de Chimie de Coordination du CNRS, 205, route de Narbonne, 31077 T oulouse
cedex 04, France. E-mail: chaudret=lcc-toulouse.fr
b L aboratoire de Physique Quantique (CNRS UMR 5626) IRSAMC, Universite Paul Sabatier,
118, route de Narbonne, 31062 T oulouse cedex 04, France
c Institut fur Chemie, Freie Universitat, Berlin, T akustrasse 3, D-14195 Berlin, Germany
d L aboratoire de Synthese et dÏElectrosynthese Organometalliques, Universite de Bourgogne,
6, Boulevard Gabriel, 21000 Dijon, France
Received (in Montpellier, France) 10th March 2000, Accepted 15th June 2000
First published as an Advance Article on the web 28th November 2000
Protonation of Cp TaH(CO) (Cp \ C H , 1a; C H But, 1b) by HBF É Et O at [78 ¡C in CH Cl a†ords
2
5
5
5
4
4
2
2 2
[Cp TaH (CO)]BF (2, 3) as mixtures of 2 isomers. The minor ones (2a, 2b) contain the known trans-dihydride
2
2
4
[Cp TaH (CO)]` cations whereas the major ones (3a, 3b) are [Cp Ta(g2-H )(CO)]BF , the Ðrst group 5
2
2
2
2
4
dihydrogen complexes. The crystal structure of the analogous complex 3a É BArf recorded at 120 K conÐrms the
4
presence of the coordinated dihydrogen ligand, which displays an HÈH separation of 1.09(2) A in agreement with
distances calculated from NMR data. Protonation of Cp TaH (SiMe Ph) by (Et O) É HBArf does not lead to an
2
2
2
2
2
4
analogous silane derivative but to the new dinuclear complex [(Cp TaH ) (l-H)](BArf ). Variable temperature
2 2
NMR studies were carried out on the dihydrogen complex [Cp Ta(H )(CO)]` (3) and its isotopomers. The high
2
4
2
2
Ðeld signal of [Cp Ta(HD)(CO)]` (3-d) shows a decoalescence at 208 K in both 1H and 2D NMR, which allows us
2
to calculate the barrier to rotation of HD (9.6 kcal mol~1). The absence of decoalescence in the signal of 3 down to
173 K and the absence of a large kinetic isotope e†ect for the classical rotation of H were demonstrated. These
2
results are understood in terms of the presence of very large exchange couplings in a non-rotating dihydrogen
ligand. The large barrier of rotation for the dihydrogen ligand in 3 was shown by DFT calculations to arise from a
transition state in which the dihydrogen ligand is only coordinated through r-donation from the HÈH bond. The
analogous phosphite and phosphine complexes MCp TaH [P(OMe) ]N` (4) and [Cp TaH (PMe Ph)]` (5) were
2
2
3
2
2
2
shown to be cis dihydrides, in agreement with DFT calculations on a model compound, to display exchange
couplings in NMR and no isotope e†ect for the classical exchange of the hydride ligands.
In the mid-eighties two major events renewed the interest in
the interaction between dihydrogen and a transition metal.
The most important was the discovery by Kubas and
coworkers that dihydrogen can coordinate to a transition
metal without being dissociated.1,2 The chemistry of dihydro-
gen complexes has developed very rapidly since then.2h7
Stable dihydrogen complexes were isolated for metals of
groups 6È10 and a dihydrogen adduct of group 11 was
observed in a matrix.5 As far as the chemistry of group 5 dihy-
drogen derivatives is concerned, in addition to the tantalum
complexes discussed in this paper and the corresponding
niobium derivatives (vide infra), the unstable complexes
CpV(CO) (g2-H )8a and CpNb(CO) (g2-H )8b were character-
lings was independently recognized by Zilm et al.12 and by
Weitekamp and coworkers,13 whereas the involvment of rota-
tional tunneling was Ðrst proposed by Limbach et al.14 The
existence and magnitude of the couplings were shown to be
favoured by a reduction of the electronic density on the metal
as was the formation of the dihydrogen state, which suggested
a link between these two phenomena. Indeed, in the two
similar complexes Cp NbH and Cp TaH displaying very
2
3
2
3
di†erent spectroscopic properties, calculations showed that
the di†erence between their electronic structures was the exis-
tence of a low-lying dihydrogen state in the former.15
In both cases group 5 polyhydrides were involved. Thus, the
spectroscopic properties of the above mentioned niobium
complex Cp NbH , characterized by two broad peaks in a
2 : 1 ratio, appeared puzzling16 and, as early as the 70Ïs,
during the course of reactivity studies, Tebbe and Parshall16a
observed that addition of AlEt to Cp NbH gave products
with large HÈH coupling constants ([100 Hz), which were
not explained at that time but are now understood in terms of
quantum mechanical exchange couplings.11 The cationic tan-
talum dihydrides17 [Cp TaH L]` [L \ P(OMe) , PMe Ph]
incorporating phosphine or phosphite ligands also displayed
large HÈH coupling constants, which were later demonstrated
to be due to quantum mechanical exchange couplings.18 This
3
2
3
2
ized in a matrix whereas the corresponding tantalum complex
was shown to be a classical dihydride.
2
3
Besides this major discovery, another new phenomenon was
reported by two groups, namely the existence of large
temperature-dependent HÈH couplings in the 1H NMR
spectra of some transition metal polyhydrides.9,10 These coup-
lings were rationalized in terms of quantum mechanical
exchange.11 The quantum mechanical origin of these coup-
3
2
3
2
2
3
2
¤ Permanent address: Laboratoire de Chimie Theorique, Faculte des
Sciences, Avenue Ibn Batouta, BP 1014, Rabat, Morocco.
DOI: 10.1039/b002073g
New J. Chem., 2001, 25, 55È62
55
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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led us to attempt the preparation of cis-dihydride complexes
containing better p-accepting ligands in the equatorial plane
in order to obtain larger exchange couplings and/or stabilize
tantalum dihydrogen complexes. For this purpose, CO was
[Cp TaH (CO)][BF ] (2a É BF , 2b É BF ) similar to the
2
2
4
4
4
complex previously characterized by one of us.19 The major
isomers (3a É BF , 3b É BF ) display a broad resonance at d
4
4
[4.98 and d [3.51 at 293 K, respectively, which only
broadens down to 178 K. The ratios 2a : 3a and 2b : 3b are
1 : 17 and 1 : 7, respectively. They remain constant through-
out the temperature range observed and even after keeping
the samples for 3 h at room temperature. The relaxation time
Ðrst chosen since Cp TaH(CO) (1) is a readily accessible
2
complex. trans-[Cp TaH (CO)]PF had previously been
2
2
6
obtained by protonation of Cp TaH(CO) in aqueous HCl at
room temperature.19 However precedents, in particular in
2
ruthenium chemistry, have shown that low temperature proto-
nation could lead to kinetic cis-isomers, which would adopt a
dihydrogen conÐguration.20 This led to the synthesis of cis-
[Cp Ta(H )(CO)]BF (3), the Ðrst thermally stable group 5
T of the hydrides (250 MHz, CD Cl ) shows a minimum of
1
2 2
350 ms for 2a and 9 ms for 3a at 178 K and of 205 ms for 2b
and 12 ms for 3b at 178 K, in agreement with the formulation
of complexes 2a and 2b as classical dihydrides and 3a and 3b
as dihydrogen derivatives. The latter are the Ðrst reported
group 5 dihydrogen complexes. Evaluation of the HÈH dis-
tance within the dihydrogen complexes 3a and 3b can be
2
2
4
dihydrogen complex.21 A similar chemistry carried out with
niobium led to the dihydrogen complexes cis-
[Cp Nb(H )(L)]`, but this time with better donor ligands
2
2
such as phosphines, phosphites and isocyanides, the carbonyl
complex being unstable.22,23 This result is in agreement with
the fact that third row transition metals will have a greater
achieved using T relaxation data. The relaxation time of
metal bound hydrides (R ), obtained in our case from mea-
surements through the inversion/recovery method, is the sum
1
T
tendancy to dissociate H because of stronger MÈH bonds
and better overlap between the metal and dihydrogen orbitals
and therefore will need better p-acceptors to accommodate
of the relaxation due to hydrideÈhydride interaction (R
)
2
HhH
and of that due to all other phenomena (R , the relaxation
O
due to dipoleÈdipole interactions with Cp, solvent, Ta, etc.).26
H . Finally, the protonation of Cp NbH in the presence of
R
can be neglected for 2a whereas R should be very
2
2
3
HhH
O
various ligands has recently been reported to lead to the trans
similar or identical for both 2a and 3a. Therefore the equa-
tions are:
dihydride derivatives [Cp NbH (L)]`.24
2
2
We report in this paper full experimental details on the
synthesis of cationic cis-tantalum dihydride complexes
[Cp Ta(H )(L)]` [L \ CO, PMe Ph, P(OMe) ] and spectro-
R (3a) \ R (3a) ] R (3a)
T
HhH
O
R (2a) B R (2a) B R (3a)
2
2
2
3
T
O
O
scopic characterization of the complexes, including the
demonstration of the hindered rotation of dihydrogen and of
its relation with quantum mechanical exchange couplings. A
signiÐcant part of this work has been reported in a prelimi-
nary form.18,21,25 We now report the X-ray crystal structure
of [Cp Ta(H )(CO)]BArf MBArf \ B[C H (CF ) ] N, the
F R (3a) \ R (3a) [ R (2a)
HhH
T
T
This is a rare case where the relaxation due to the hydrideÈ
hydride interaction can be estimated with precision. Since we
know that the dihydrogen molecule is blocked at low tem-
perature, an approximate equation26 allows us to calculate an
HÈH separation of 1.06 ^ 0.05 A for 3a and 1.12 ^ 0.05 A for
3b. These distances are in agreement with the presence of
stretched dihydrogen ligands in the coordination sphere of
tantalum. This was unexpected for a cationic early transition
metal complex but is in agreement with the good electronic
transfer from tantalum, a third row transition metal. It is
interesting to note the variation of the relaxation time of the
hydride signals of both isomers when substituting C H by
2
2
4
4
6
3
3 2 4
Ðrst crystal structure of a group 5 dihydrogen complex, proto-
nation of related species and new calculations using the DFT/
B3LYP method on the full system.
Results and discussion
Dihydrogen carbonyl complexes
5
5
Protonation of Cp TaH(CO) (Cp \ C H , 1a;19 C H But,
C H But. This is due to the variation of the electronic proper-
2
5
5
5
4
5 4
1b) by stoichiometric amounts of HBF É Et O in CH Cl at
ties of the Cp ligands: the more electron-releasing ligand
4
2
2 2
[78 ¡C a†ords rapidly [Cp TaH (CO)]BF as two isomeric
classes of compounds (see Scheme 1). The minor isomers were
C H But induces a lengthening of the HÈH bond.
2
2
4
5 4
Upon carrying out the protonation of 1a with DBF É D O,
4
2
identiÐed by their spectroscopic properties as trans-
the expected 1 : 1 : 1 triplet centred at d [5.18 (see Fig. 1) was
Scheme 1 Protonation reactions of Cp TaH(CO) and Cp TaH(PR ) [PR \ P(OMe) , PMe Ph].
2
2
3
3
3
2
56
New J. Chem., 2001, 25, 55È62

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ppm, which remains as a singlet down to 183 K. The signal for
coordinated HD is not very clear on the spectrum but can
easily be visualized using the INEPT 2D/1H technique. A
doublet is observed at [5.07 ppm (J ) ca. 27.5 Hz) at 298
HhD
K, which splits at 183 K into two doublets at [5.10 and
[5.75 ppm. This demonstrates the decoalescence of the HD
signal as observed by 2D NMR. The presence of a single high-
Ðeld line for 3a (1H NMR) and 3a-d (2D NMR) and of a
2
decoalescence for 3a-d (1H and 2D NMR) cannot be attrib-
uted to a kinetic HH/HD/DD isotope e†ect since 3a-d would
2
be expected to show a similar behaviour as 3a-d but at higher
temperature. These experiments therefore demonstrate the
presence of very large HÈH (DÈD) couplings in
[Cp Ta(H )(CO)]` and [Cp Ta(D )(CO)]`. The origin of
2
2
2
2
these couplings is quantum mechanical in nature and involves
rotational tunneling of the coordinated dihydrogen ligand.
Tunnelling is possible for two similar particles but not for dif-
ferent ones such as H and D, hence explaining the decoale-
scence observed for the HD isotopomer.
In order to obtain crystals of these new complexes, the
protonation of 1 was carried out under conditions similar to
those used for the NMR studies ([78 ¡C, CH Cl ) but with
2
2
(Et O) É HBArf [Arf \ (CF ) C H ].29 Addition of pentane
2
2
4
3 2 6 3
Fig. 1 High-Ðeld 1H NMR spectra of [Cp Ta(HD)(CO)]` (3a-d) at
to the reaction solution leads to the crystallization of the dihy-
drogen complexes (3a É BArf , 3b É BArf ) in good yields (ca.
2
various temperatures (250 MHz, CD Cl ). Signal ““cÏÏ represents the
2
2
dihydrogen ligand of 3a; signals ““aÏÏ and ““bÏÏ are for the endo and exo
4
4
60%).
isomers of 3a-d.
The crystal structure of 3a É BArf was solved by X-ray dif-
fraction at 120 K and converged to a satisfactory agreement
4
observed for 3a-d, namely [Cp Ta(g2-HD)(CO)][BF ]. The
factor (R \ 0.052 and R \ 0.062). It consists (see Fig. 2 and
2
4
w
J
value is 27.5 Hz in agreement with a dihydrogen struc-
Tables 1 and 2) of a classical ““sandwichÏÏ structure with the
HhD
ture. Using the correlation established by Morris et al.27 or
that of Luther and Heinekey,28 it is possible to estimate the
HÈH distance knowing the HÈD coupling within a dihydro-
gen complex. The value obtained is 0.96 A, in fair but not
carbonyl and dihydrogen ligands lying in the equatorial plane.
The TaÈC and CÈO distances within the coordinated carbonyl
group are respectively 2.063(12) and 1.127(13) A, slightly but
not signiÐcantly longer than those found for 1 [respectively
1.94(4) and 1.21(4) A], in agreement with the cationic nature of
excellent agreement with T calculations. Upon lowering the
1
temperature, the 1 : 1 : 1 triplet signal observed for the hydride
3a É BArf .30 Similarly, the TaÈH distances [1.78(4) and 1.81(4)
4
coupled to the deuteride at d [5.17 (J
HhD
at 208 K and transformed into two triplets at d [5.78 and d
27.5 Hz) coalesced
A] are longer in the present case than in 1 (1.45 A, Ðxed
value), as expected for a dihydrogen complex, and the HÈH
distance [1.09(2) A] matches well the spectroscopic data. In
particular this HÈH distance is in agreement with the stretch-
ing of the HÈH bond.
[5.14 (J 27.5 Hz for both) due to the non-rotating HD
HhD
molecule in which the hydrogen atom is located either next to
CO or opposite to it. The energy of activation for the rotation
of HD was calculated to be 9.6 kcal mol~1 (see Fig. 1). This
value is high and was unexpected for the rotation barrier of a
coordinated HD molecule. It seems, however, typical for
group 5 dihydrogen complexes since the barrier found for the
Tantalum dihydride complexes containing phosphorus donor
ligands
The corresponding complexes incorporating phosphorus
donor ligands [PMe Ph and P(OMe) ] had been previously
niobium
complexes
[Cp Nb(HD)(PMe Ph)]`22
and
2
2
2
3
[Cp Nb(HD)(NCR)]` (R \ But, Cy, 2,6-Me C H )23 are
2
2 6 3
respectively 11.0 and 10.0 kcal mol~1.
It is surprising, however, that no decoalescence is observed
for the high-Ðeld signal of 3a, b. Two explanations could
account for this observation: (i) a large kinetic isotope e†ect
or (ii) very large couplings between the hydrogens of the coor-
dinated dihydrogen molecule. In the latter case, when the
HÈH coupling constant J becomes large compared to the
chemical shift di†erence *l (J/*l [ 10), the expected AB type
spectrum would collapse into a single line located at a chemi-
cal shift intermediate between that of the hydrogen atoms.
The determination of the individual chemical shifts of the
hydrogens located respectively next to the carbonyl group or
opposite to the carbonyl group was made possible by the
decoalescence of the high-Ðeld signal of the HD isotopomer.
In an experiment carried out at 250 MHz, *l \ 160 Hz and
therefore the coupling between the hydrogens must be larger
than 1600 Hz at 173 K. However, the same experiment was
carried out in a freon mixture at 130 K and 500 MHz. Since
no decoalescence was again observed this gives a minimum
value of the HÈH coupling constant of 3200 Hz at 130 K.
The 2D NMR spectra (CD Cl , 61.422 MHz) of a mixture
2
2
of 3a-d and [Cp Ta(D )(CO)]` (3a-d ) show at 298 K the
Fig. 2 Molecular structure of 3a É BArf with 50% probability ellip-
2
2
2
4
presence of a signal attributed to coordinated D at [5.67
soids for the non-hydrogen atoms.
2
New J. Chem., 2001, 25, 55È62
57

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Table 1 Crystal data, data collection and structure reÐnement of 3a É BArf É 1.5CH Cl
4
2
Chemical formula
Molecular weight
[C
1331.77
H
OTa][B(C H )(CF ) ] É 1.5CH Cl
3 2 4 2 2
11 12
6
3
Crystal system
Space group
Monoclinic
P2 /n
1
a/A
b/A
c/A
b/¡
13.152(2)
18.720(3)
20.918(3)
99.40(2)
5067(2)
4
120
24.16
39 337
7545
0.044
0.052
0.062
U/A
Ž
3
Z
T /K
k/cm~1
No of measured reÑections
No of independent reÑections
Merging R value
R
R
w
prepared by Moise et al.17 The cis-dihydrides were prepared
by protonation of the corresponding monohydrides (see
Scheme 1) and shown to exhibit quantum mechanical
exchange couplings, low in magnitude and of the same sign as
the magnetic ones, in contrast to similar trihydride systems.18
For example, in the case of cis-MCp TaH [P(OMe) ]N` (4),
kcal mol~1, as obtained by the lineshape analysis shown on
Fig. 3. The spectra also show a slow transformation of 4 into
the trans form, especially at high temperatures, probably
linked to H/D exchange with the solvent and D O present in
2
the reaction mixture through deprotonation/protonation pro-
cesses since, after heating to 375 K and cooling, 4 trans-d is
the major isotopomer present in solution.
These results therefore conÐrm that (i) a classical Ñuxional
process is operative in, so far, every system displaying
quantum exchange and (ii) no kinetic isotope e†ect is present
for the classical hydride/hydride exchange process.
2
2
3
the couplings were found to vary between 76.1 Hz at 293 K
and 12.5 Hz at 178 K, whereas for the phosphine complex
cis-[Cp TaH (PMe Ph)]` (5), they vary between 15.6 Hz at
2
2
2
310 K and 4.6 Hz at 193 K. The values of the di†erent coup-
lings are clearly related to the respective r-donor and p-
acceptor abilities of the di†erent ligands. This point will be
developed in the theoretical section of the present paper.
However, having demonstrated that large HÈH couplings
rather than a kinetic isotope e†ect is at the origin of the
apparent absence of decoalescence of the high-Ðeld signal of 3,
it was of interest to determine if the absence of kinetic isotope
e†ect is a general property of such dihydrides. We therefore
measured the 1H NMR spectra of 4 and its isotopomer 4-d at
variable increasing temperatures. Two high-Ðeld signals are
observed in 1H NMR at room temperature (dioxane-d , 250
8
MHz; see Fig. 3) corresponding to the localization of the
hydride either next to the phosphite ligand or opposite to it,
respectively at d [2.10 (J
PhH
coupling constant was found to be ca. 1.5 Hz,
89.1 Hz) and [0.94 (J
9.5
PhH
Hz). The J
HhD
in agreement with a classical dihydride formulation for cis-
MCp Ta(H) [P(OMe) ]N` (4), for which an HÈH separation
2
2
3
of 1.67 A was calculated using T minimum data (87 ms at
1
180 K, 250 MHz). The hydride signals for the isotopomers 4
and 4-d both show a coalescence at the same temperature of
about 360 K (250 MHz, dioxane-d ), indicating the absence of
8
a kinetic HH/HD isotope e†ect for the hydride/hydride
exchange. The process is characterized by a barrier of 16.6
Table 2 Selected bond distances (A) and angles (¡) for 3a É BArf
(esdÏs in parentheses refer to the last signiÐcant digit)a
4
H(1)ÈH(2)
TaÈH(2)
TaÈC(2)
TaÈC(4)
TaÈC(6)
TaÈC(8)
TaÈC(10)
OÈC(11)
TaÈL1
1.09(2)
1.78(4)
2.37(1)
2.32(1)
2.37(1)
2.36(1)
2.37(1)
1.127(13)
2.0305
TaÈH(1)
TaÈC(1)
TaÈC(3)
TaÈC(5)
TaÈC(7)
TaÈC(9)
TaÈC(11)
TaÈL2
1.81(4)
2.357(11)
2.357(11)
2.350(11)
2.32(1)
2.344(12)
2.063(12)
2.0358
Fig. 3 High-Ðeld 1H NMR spectra of the mixture cis-
MCp Ta(H) [P(OMe) ]N` (4), cis-MCp Ta(H)(D)[P(OMe) ]N` (4-d),
C(11)ÈTaÈH(2)
TaÈC(11)ÈO
64.1(3)
179.2(1)
C(11)ÈTaÈH(1)
H(1)ÈTaÈH(2)
98.9(3)
35.4(9)
2
2
3
2
3
trans-MCp Ta(H) [P(OMe) ]N` (4 trans) and trans-MCp Ta(H)(D)
2
2
3
2
[P(OMe) ]N` (4 trans-d) at various temperatures (250 MHz, dioxane-
a L1 \ centroid of C(1)ÈC(5), L2 \ centroid of C(6)ÈC(10).
3
d ).
8
58
New J. Chem., 2001, 25, 55È62

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Table 3 Optimized geometrical parameters for the cis- and trans-
[Cp TaH (CO)]` dihydride complexes.a Distances are in A and
angles in ¡
Protonation of a tantalum hydrido silyl complex
2
2
Since the protonation of 1 leads to a complex characterized as
the Ðrst group 5 dihydrogen derivative, it was of interest to
study other systems that could lead to other novel structures.
The protonation of Cp TaH 24 could produce dihydrido
trans(A)
cis(B)
B3LYP
B3LYP
BLYP
2
3
dihydrogen or bis(dihydrogen) complexes while the proto-
TaÈH
a
1.759
1.759
3.191
2.161
1.142
2.110
130.2
65.1
1.735
1.778
1.504
2.135
1.146
2.131
50.7
113.3
62.6
138.8
176.6
1.750
1.791
1.516
2.133
1.160
2.138
50.7
113.0
62.3
141.0
179.9
nation of Cp TaH (SiMe Ph)31 could lead to a complex con-
2
2
2
TaÈH
b
taining coordinated dihydrogen, coordinated silane or both.
The reaction with Cp TaH did not lead to a dihydrogen
H ÈH
a
b
TaÈC
2
3
complex. However, protonation of Cp TaH (SiMe Ph) by
(Et O) É HBArf
CÈO
2 2 2
yielded
a
new dinuclear complex
TaÈL
2
2
4
1
H ÈTaÈH
[(Cp TaH ) (l-H)](BArf ) (6; Scheme 2), isolated as white
a
b
2
2 2
4
H ÈTaÈC
crystals in good yield (59%). The 1H NMR spectrum of this
a
H ÈTaÈC
65.1
139.8
180.0
complex (200 MHz, CD Cl ) shows an AX type spectrum at
b
2
2
4
HhH
L ÈTaÈL
high Ðeld: a quintet at [17.16 ppm (1H, J
9.8 Hz) and a
1
2
TaÈCÈO
doublet at 0.82 ppm (4H), corresponding respectively to one
bridging and four terminal hydrides. This indicates the forma-
tion of a dinuclear complex linked by only one bridging
hydride. The mechanism of formation of this complex is
unknown.
a See Fig. 4 for labeling of the hydrides. L and L stand for the Cp
1
2
centres.
tion of the trans equatorial hydride ligands, which are bent
away from the Cp ligand with an H ÈTaÈH angle of 130.2¡. A
DFT calculations on [Cp TaH L]‘ with L = CO, PH
3
2
2
a
b
similar geometry with an HÈTaÈH angle of 129.8¡ has been
found at the MP2 level in the d0 complex
[TaH (OH) (CO)(PH )]` by Bayse and Hall.32 The reason
Four isomers have been located on the potential energy
surface when L \ CO. They are depicted in Fig. 4, and selec-
ted sets of optimized bond lengths and angles are listed in
Tables 3 and 4. Isomers A, B and C have been found to be
minima on the surface. The most stable point on the surface is
isomer A with the carbonyl ligand lying in a central position.
2
2
3
for this distortion is the strong p-acceptor property of the CO
ligand, which polarizes the Ta d orbital used to coordinate the
hydrides. The calculated TaÈH distance of 1.759 A is typical of
tantalumÈhydride distances and compares very well with the
MP2 value of 1.756 A obtained in the previous complex.
This isomer with C symmetry is characterized by a distor-
2v
Isomers B and C are of C symmetry with the two hydrogen
s
atoms on the same side with respect to CO, in the equatorial
plane. The characteristic geometrical feature of isomer B is the
presence of two inequivalent TaÈH bonds with a distance
between the two hydrides of 1.504 A. It can therefore be
described as a classical cis-dihydride complex. It can be
noticed that the in-plane p* CO orbital polarizes the two
TaÈH bonds di†erently leading to one bond (TaÈH ) being
a
slightly shorter than in isomer A and the other (TaÈH ) being
b
slightly longer (by about 0.02 A).
Isomer C is clearly a dihydrogen complex. The (g2-H )
2
coordination mode is conÐrmed by a signiÐcant lengthening
of the H ÈH bond (0.922 A vs. 0.74 A in free dihydrogen), and
a
b
TaÈH and TaÈH bond distances (1.834 and 1.858 A,
a
b
Scheme 2 Protonation reaction of Cp TaH (SiMe Ph).
respectively) much longer than in isomer A and B. The opti-
mized geometry of isomer C closely resembles that found by
2
2
2
X-ray di†raction for 3a É BArf . As expected,33 DFT calcu-
4
lations give metalÈhydrogen bond distances that are too long.
The H ÈH bond distance is calculated to be smaller than the
a
b
Table
4
Optimized geometrical parameters for the [Cp Ta(g2-
2
H )(CO)]` complexes.a Distances are in A and angles in ¡
2
Isomer C
Isomer D
B3LYP
B3LYP
BLYP
EXP
TaÈH
TaÈH
1.834
1.858
0.922
2.103
1.151
2.102
28.9
102.6
73.3
140.1
179.1
1.842
1.865
0.940
2.104
1.165
2.165
29.4
102.6
73.2
140.2
179.9
1.81(4)
1.78(4)
1.09(2)
2.063(12)
1.127(13)
È
35.4(9)
98.9(25)
64.1(26)
È
2.039
2.039
0.787
2.078
1.156
2.091
22.3
90.5
90.5
140.6
176.5
a
b
b
H ÈH
a
TaÈC
CÈO
TaÈL
1
H ÈTaÈH
a
b
H ÈTaÈC
a
H ÈTaÈC
b
L ÈTaÈL
1
2
TaÈCÈO
179.2(11)
Fig. 4 Geometries of the stationary points located on the potential
energy surface of [Cp TaH (CO)]`. Hydrogen atoms of the Cp ring
are omitted for clarity.
a See Fig. 4 for labeling of the hydrides. L and L stand for the Cp
centres.
1
2
2
2
New J. Chem., 2001, 25, 55È62
59

View Article Online
Table 6 Optimized geometrical parameters for [Cp TaH (PH )]`
X-ray experimental one by about 0.17 A. The use of the non-
hybrid functional BLYP does not alter this result. However,
the H ÈH calculated value is in an overall good agreement
2
2
3
complexes.a Distances are in A and angles in ¡
Dihydride complexes
g2-H complex
a
b
2
with the values deduced from NMR experiments using T
data, and conÐrms an important stretching of the coordinated
HÈH bond.
1
trans (A)
cis (B)
Perpendicular (D)
TaÈH
TaÈH
1.751
1.763
3.191
1.428
1.431
2.114
2.599
98.2
1.724
1.762
1.584
1.426
1.431
2.123
2.638
97.7
98.1
54.0
138.3
119.3
65.2
2.045
2.045
0.787
1.431
1.440
2.075
2.541
99.0
96.8
22.2
141.1
88.5
88.5
The relative energies of the B3LYP-optimized geometries
are presented in Table 5. The three isomers of
[Cp TaH (CO)]` are within an energy range of 4 kcal mol~1.
a
b
b
H ÈH
a
PÈH
1
2
2
PÈH
2
Isomers A and C are separated by only 2.4 kcal mol~1. They
correspond to the two isomers 2a and 3a a†orded by proto-
TaÈL
1
TaÈP
nation of Cp TaH(CO) and identiÐed by NMR spectroscopy.
2
H ÈPÈH
This result is in agreement with experimental results demon-
2
3
2
H ÈPÈH
99.0
1
strating that 2a is the thermodynamic isomer and 3a the
kinetic isomer.
H ÈTaÈH
130.5
138.2
62.3
68.2
111.0
a
b
L ÈTaÈL
1
2
H ÈTaÈP
Additionally, the transition state structure of the rotation of
H in isomer C was optimized by forcing the H fragment to
a
H ÈTaÈP
b
2
2
L ÈTaÈP
105.0
105.7
be perpendicular to the plane of symmetry of the molecule.
This structure corresponds to isomer D in Fig. 4, and its
structural parameters can be found in Table 4. The rotational
barrier is calculated to be 12.7 kcal mol~1, in good agreement
with that obtained from NMR data (9.6 kcal mol~1). This
high value is due to the complete loss of back-donation from
1
a See Fig. 5 for labeling of the hydrides. L and L stand for the Cp
centres.
1
2
dihydride structure in which the hydride ligands bend toward
the metal centre into the r* orbital of H in the rotated struc-
2
the phosphine. The calculated value of the H ÈTaÈH bond
ture. This statement is corroborated by the change in the HÈH
a
b
angle (130.5¡) is of the same order of magnitude as in the
trans-dihydride carbonyl complex, indicating that the distor-
tion of the hydrides is characteristic of the d-orbital occu-
pancy of the metal. In contrast to the case with L \ CO, the
phosphine complex has only one isomeric form in which the
distance, from 0.922 A in isomer C to 0.787 A in the transition
structure, while the TaÈH bond lengths increase by about
0.2 A.
For the phosphine complex [Cp TaH (PH )]`, three sta-
2
2
3
tionary points of interest have been identiÐed on the potential
energy surface. The B3LYP-optimized geometries of three
isomers are featured in Fig. 5, and a set of selected geometrical
parameters can be found in Table 6. As for the carbonyl com-
plexes, the most stable isomer A corresponds to a trans-
two hydrides occupy a lateral position with respect to PH .
3
Isomer B can be formulated as a classical cis-dihydride with
an H ÈH distance of 1.584 A. This value is in good agreement
a
b
with the HÈH separation of 1.67 A calculated from NMR data
for cis-MCp TaH [P(OMe) ]N`. The reason for the absence of
2
2
3
an g2-H isomer in this compound is that the PH ligand acts
2
3
Table 5 Relative energies (in kcal mol~1) of the various isomers of
[Cp TaH L]` at the DFT/B3LYP level of calculation
as a weak p-acceptor. Therefore, the delicate balance between
L and H for back-bonding is in favour of H , which leads to
2
2
2
2
the breaking of the HÈH bond.
L \ CO
L \ PH
3
Finally, the barrier to hydride/hydride exchange in isomer
B has been calculated by optimizing the geometry of the tran-
Dihydride complexes
trans (A)
sition structure in which the H fragment is perpendicular to
0.0
4.0
0.0
9.8
2
cis (B)
the plane of symmetry (see isomer D in Fig. 5). This isomer
g2-H complexes
displays the same geometrical features as the corresponding
transition state in the carbonyl complex, that is a weakly
2
Coplanar (C)
2.4
È
Perpendicular (D)
15.1
25.8
bonded H molecule. The height of the barrier is calculated to
2
be 16.0 kcal mol~1, in good agreement with the value of 16.6
kcal mol~1 obtained by lineshape analysis for hydride/hydride
exchange in complex 4.
Conclusions
We report in this article the Ðrst X-ray crystal structure of a
group 5 dihydrogen complex. The complex is formally d2 but
accomodates a stretched dihydrogen ligand, hence suggesting
an efficient back-bonding from the metal. The dihydrogen
ligand is very sensitive to small variations of the electron
density on the metal as found experimentally and modelled by
calculations. Thus, replacing CO by a phosphine ligand
induces the formation of a cis-dihydride complex.
It has not been possible to extend this study to the prep-
aration of the analogous silane complexes but the protonation
of the dihydridosilyl complex has led to an interesting dinu-
clear tantalum derivative containing a single hydride bridge,
[(Cp TaH ) (l-H)](BArf ). This complex results formally from
2
2 2
4
the coordination of Cp TaH to the cationic Cp TaH ` frag-
2
3
2
2
ment and therefore can be viewed as another example of a
r-bond tantalum complex, in the present case a TaÈH bond.
The most original result of this work concerns the
presence of quantum mechanical exchange couplings in
Fig. 5 Geometries of the stationary points located on the potential
energy surface of [Cp TaH (PH )]`. Hydrogen atoms of the Cp ring
2
2
3
are omitted for clarity.
60
New J. Chem., 2001, 25, 55È62

View Article Online
the
cis-dihydride
or
dihydrogen
derivatives.
In
The structure was solved by direct methods SIR92,35 and
MCp TaH [P(OMe) ]N` (4), the exchange couplings result
reÐned by least-squares procedures on F (CRYSTALS).36
2
2
3
obs
from the possibility of the two hydrogens to exchange through
a dihydrogen transition state located 16 kcal mol~1 above the
ground state. This transition state is the same as that found
for the dihydrogen rotation in the carbonyl complexes. In this
transition state, the dihydrogen ligand is perpendicular to the
equatorial plane and only coordinated through r-donation
from the HÈH bond. This is clear when considering the HÈH
distance of 0.787 A calculated for the transition state of the
carbonyl complex. This absence of back-donation explains the
height of the barrier (9.6 kcal mol~1), which is very unusual
for dihydrogen rotation and only observed in d2 systems.
All hydrogen atoms were located on a di†erence Fourier map,
but they have been introduced in the calculation in idealized
positions, with an isotropical thermal parameter Ðxed at 20%
higher than those of the carbon to which they are bound. The
hydrides H(1) and H(2) have been isotropically reÐned. All
non-hydrogen atoms were anisotropically reÐned and in the
last cycles of reÐnement using a weighting scheme. The
drawing of the molecule is performed with CAMERON37
with 50% probability displacement ellipsoids for non-
hydrogen atoms.
CCDC reference number 440/202. See http://www.rsc.org/
suppdata/nj/b0/b002073g/ for crystallographic Ðles in .cif
format.
Experimental
Computational details
Syntheses
All calculations were performed with the GAUSSIAN 98
program.38 For tantalum, the core electrons were represented
by a relativistic small-core pseudopotential derived using the
DurandÈBarthelat method.39 The thirteen electrons corre-
sponding to the 5s, 5p, 5d and 6s atomic orbitals were
described by a (8s, 6p, 6d) primitive set of Gaussian functions
contracted to [7s, 5p, 3d]. Standard pseudopotentials devel-
oped in Toulouse40 were used to describe the atomic cores of
all other atoms (P, C, O). A double-zeta plus polarization
valence basis set was employed for each atom (d-type function
exponents were 0.45, 0.80 and 0.85, respectively). For hydro-
gen, a primitive (4s) basis set contracted to [2s] was used. A
p-type polarization function (exponent 0.9) was added for the
two hydrogen atoms directly bound to tantalum (H and H ).
All stationary points of interest were located at the B3LYP
level of theory, a density functional theory (DFT) type of cal-
culation based on hybrid functionals. Analytical Ðrst deriv-
atives of the energy were used to optimize geometrical
parameters and frequency calculations were performed to
determine the nature of the stationary points.
[Cp Ta(H )(CO)]BArf
(3a). To
a
solution of
2
2
4
Cp Ta(H)(CO) (75 mg, 0.22 mmol) in 4 mL of CH Cl at
2
2 2
[78 ¡C was added a solution of (Et O) É HBArf (222.6 mg,
2
2
4
0.22 mmol) in 1 mL of CH Cl via canula. The reaction
2
2
mixture was stirred for 30 min. Pentane (6 mL) was slowly
added and di†used into the resulting yellow solution. After 2
days at [30 ¡C, the complex was isolated as yellow crystals.
Yield: 60%. 1H-NMR (200 MHz, CD Cl ): d 7.74 (br, 8H,
2
4
2
H-o of BArf ), 7.58 (s, 4H, H-p of BArf ), 5.57 (s, 10H, C H ),
4
5 5
[5.05 [s, 2H, Ta(H )]. Anal. calc. for C
H
BF OTa: C
2
43 24 24
42.88, H 1.99; found: C 42.44, H 1.77%.
[(C H But) Ta(H )(CO)]BArf (3b). To
a
solution of
5
4
4
2
2
4
(C H But) Ta(H)(CO) (50 mg, 0.11 mmol) in 3 mL of CH Cl
a
b
5
2
2 2
at [78 ¡C was added a solution of (Et O) É HBArf (111.9
2
2
4
mg, 0.11 mmol) in 1 mL of CH Cl via canula. The reaction
2
2
mixture was stirred for 30 min. Pentane (5 mL) was slowly
added and di†used into the resulting yellow solution. After 2
days at [30 ¡C, the complex was isolated as yellow crystals.
Yield: 56%. 1H-NMR (200 MHz, CD Cl ): d 7.72 (br, 8H,
2
2
H-o of BArf ), 7.57 (s, 4H, H-p of BArf ), 5.67 (m, 2H, C H ),
4
4
4
5 4
5 4
5.60 (m, 2H, C H ), 5.40 (m, 2H, C H ), 5.20 (m, 2H, C H ),
Acknowledgements
5
4
5
1.24 (s, 18H, But), [4.01 [s, 2H, Ta(H )]. Anal. calc. for
2
This work was supported by the European Union, Brussels,
via the Human Capital & Mobility Network ““Localization
and Transfer of HydrogenÏÏ, the Deutsche Akademische Aus-
tauschdienst, Bonn-Bad Godesberg, within the programme
PROCOPE (H.-H. L., B. C.), the CNRS (B. C., S. S.-E.) and
the Fonds der Chemischen Industrie, Frankfurt (S. U., H.-H.
L.). The authors thank Madame Perret for technical assist-
ance.
C
H
BF OTa: C 46.52, H 3.07; found: C 46.04, H 2.81%.
51 40 24
[(Cp TaH ) (l-H)]BArf
(6). To
a
solution of
2
2 2
4
Cp TaH (SiMe Ph) (36 mg, 0.08 mmol) in 3 mL of CH Cl at
2
2
2
2 2
[78 ¡C was added a solution of (Et O) É HBArf (81.1 mg,
2
2
4
0.08 mmol) in 1 mL of CH Cl via canula. The reaction
2
2
mixture was stirred for 30 min. Pentane (5 mL) was slowly
added and di†used into the resulting colourless solution. After
resting overnight at [30 ¡C, the complex was isolated as
white crystals. Yield: 59%. 1H-NMR (200 MHz, CD Cl ): d
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