Silyl hydrides of tantalum supported by cyclopentadienyl-imido ligand sets: Syntheses, X-ray, NMR, and DFT studies

Article abstract of DOI:10.1021/om800553y

Reactions of the imido complex Cp(ArN)Ta(PMe₃)₂ (1, Ar = 2,6-diisopropylphenyl) with silanes afford the silyl hydrides Cp(ArN)Ta(PMe₃)(H)(SiR_nCl_3-n) (2b-e) and Cp(ArN)Ta(PMe₃)(H)(SiPhMeH) (2a) as the first kinetic products. However, the hydride compounds Cp(ArN)Ta(PMe₃)(H)(SiR _nCl_3-n) are metastable and, first, rearrange in the presence of phosphine to the chlorides Cp(ArN)Ta(PMe₃)(Cl)(SiHR _nCl_2-n) (5) and then decompose to Cp(ArN)Ta(PMe ₃)(Cl)(H) (4) and eventually to Cp(ArN)Ta(PMe₃)Cl ₂ (3). Complexes with a smaller Ar' substituent at nitrogen (Ar′ = 2,6-dimethylphenyl) react faster, as do more Lewis acidic silanes. The occurrence of interligand hypervalent interactions in the tantalum complexes Cp(ArN)Ta(PMe₃)(H)(SiR_nCl_3-n) has been revealed by X-ray structure analysis, DFT calculations, and the experimental determination of the sign of the coupling constant J(Si-H). The J(Si-H) was found to be negative for Cp(ArN)Ta(PMe₃)(H)(SiMe_nCl _3-n) (J(Si-H) = -40 Hz for n = 1; J(Si-H) = -50 Hz for n = 0), indicative of the presence of Si-H bonding, but positive for Cp(ArN)Ta(PMe ₃)(H)(SiMeHPh) (J(Si-H) = +14 Hz), suggesting the absence of direct Si-H interactions. A DFT study of the mechanism of silane coupling with the model imido complex Cp(MeN)Ta(PMe₃)₂ established the feasibility of the direct addition of silanes HSiMe_nCl_3-n (n = 1-3) to the imido group to give the adduct Cp(MeN{→SiR ₃-H})Ta(PMe₃)₂, as previously found in the related niobium chemistry.

Full text of DOI:10.1021/om800553y

5968
Organometallics 2008, 27, 5968–5977
Silyl Hydrides of Tantalum Supported by Cyclopentadienyl-imido
Ligand Sets: Syntheses, X-ray, NMR, and DFT Studies
Stanislav K. Ignatov,† Nicholas H. Rees,‡ Alexei A. Merkoulov,§ Stuart R. Dubberley,‡
Alexei G. Razuvaev,† Philip Mountford,*,‡ and Georgii I. Nikonov*,§,
Department of Chemistry, Nizhny NoVgorod State UniVersity, Gagarin AVenue 23,
603600 Nizhny NoVgorod, Russian Federation, Chemistry Research Laboratory, UniVersity of Oxford,
Mansfield Road, Oxford OX1 3TA, U.K., Department of Chemistry, Moscow State UniVersity, Vorob’eVy
Gory, 119992, Moscow, Russian Federation, and Department of Chemistry, Brock UniVersity, Glenridge
AVenue 500, St. Catharines, ON L2S 3A1, Canada
ReceiVed June 13, 2008
Reactions of the imido complex Cp(ArN)Ta(PMe3)2 (1, Ar ) 2,6-diisopropylphenyl) with silanes afford
the silyl hydrides Cp(ArN)Ta(PMe3)(H)(SiRnCl3-n) (2b-e) and Cp(ArN)Ta(PMe3)(H)(SiPhMeH) (2a)
as the first kinetic products. However, the hydride compounds Cp(ArN)Ta(PMe3)(H)(SiRnCl3-n) are
metastable and, first, rearrange in the presence of phosphine to the chlorides Cp(ArN)Ta-
(PMe3)(Cl)(SiHRnCl2-n) (5) and then decompose to Cp(ArN)Ta(PMe3)(Cl)(H) (4) and eventually to
Cp(ArN)Ta(PMe3)Cl2 (3). Complexes with a smaller Ar′ substituent at nitrogen (Ar′ ) 2,6-dimethylphenyl)
react faster, as do more Lewis acidic silanes. The occurrence of interligand hypervalent interactions in
the tantalum complexes Cp(ArN)Ta(PMe3)(H)(SiRnCl3-n) has been revealed by X-ray structure analysis,
DFT calculations, and the experimental determination of the sign of the coupling constant J(Si-H). The
J(Si-H) was found to be negative for Cp(ArN)Ta(PMe3)(H)(SiMenCl3-n) (J(Si-H) ) -40 Hz for n )
1; J(Si-H) ) -50 Hz for n ) 0), indicative of the presence of Si-H bonding, but positive for
Cp(ArN)Ta(PMe3)(H)(SiMeHPh) (J(Si-H) ) +14 Hz), suggesting the absence of direct Si-H interactions.
A DFT study of the mechanism of silane coupling with the model imido complex Cp(MeN)Ta(PMe3)2
established the feasibility of the direct addition of silanes HSiMenCl3-n (n ) 1-3) to the imido group to
give the adduct Cp(MeN{fSiR3-H})Ta(PMe3)2, as previously found in the related niobium chemistry.
We4,5 and others6 have been studying the application of the
Introduction
imido ligand (RN)2-, which is isolobal to the ubiquitous
Early transition metal silyl complexes in nonmetallocene
environments1 have recently received significant attention. Most
of this interest stems from their relevance to catalytic transfor-
mations of organosilanes2 such as dehydrogenative polymeri-
zation, and uses as potential precursors to M/N/Si ceramics,
which have useful applications in microelectronics as diffusion
barriers.3
cyclopentadienide ligand Cp-,7 to the design of metallocene-
(5) (a) Nikonov, G. I.; Mountford, P.; Ignatov, S. K.; Green, J. C.; Cooke,
P. A.; Leech, M. A.; Kuzmina, L. G.; Razuvaev, A. G.; Rees, N. H.; Blake,
A. J.; Howard, J. A. K.; Lemenovskii, D. A. Dalton Trans. 2001, 2903. (b)
Nikonov, G. I.; Mountford, P.; Dubberley, S. R. Inorg. Chem. 2003, 42,
258. (c) Ignatov, S. K.; Rees, N. H.; Dubberley, S. R.; Razuvaev, A. G.;
Mountford, P.; Nikonov, G. I. Chem. Commun. 2004, 952.
* Corresponding authors. (G.I.N.) Tel: (905) 6885550, ext 3350. Fax:
+1 (905) 6829020. E-mail: gnikonov@brocku.ca; (P.M.) philip.mountford@
chemistry.oxford.ac.uk.
(6) Only a few examples of silane additions to an imido group are
known: (a) Burckhardt, U.; Casty, G. L.; Tilley, T. D.; Woo, T. W.;
Rothlisberger, U. Organometallics 2000, 19, 3830. (b) Gountchev, T. I.;
Tilley, T. D. J. Am. Chem. Soc. 1997, 119, 12831. For an apparent silyl
migration to an imido group see: (c) Cameron, T. M.; Ortiz, C. G.; Ghiviriga,
I.; Abboud, K. A.; Boncella, J. M. J. Am. Chem. Soc. 2002, 124, 922–923.
(7) Williams, D. S.; Schofield, M. H.; Anhaus, J. T.; Schrock, R. R.
J. Am. Chem. Soc. 1990, 112, 6728. (b) Glueck, D. S.; Green, J. C.;
Michelman, R. I.; Wright, I. N. Organometallics 1992, 11, 4221. (c)
Williams, D. N.; Mitchell, J. P.; Pool, A. D.; Siemeling, U.; Clegg, W.;
Hockless, D. C. R.; O’Neil, P. A.; Gibson, V. C. J. Chem. Soc., Dalton
Trans. 1992, 739. (d) Williams, D. S.; Schofield, M. H.; Schrock, R. R.
Organometallics 1993, 12, 4560. (e) Sundermeyer, J.; Runge, D. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1255. (f) Gibson, V. C. J. Chem. Soc.,
Dalton Trans. 1994, 1607. (g) Wigley, D. E. Prog. Inorg. Chem. 1994, 42,
239.
(8) Nikonov, G. I.; Kuzmina, L. G.; Vyboishchikov, S. F.; Lemenovskii,
D. A.; Howard, J. A. K. Chem.-Eur. J. 1999, 5, 2497. (b) Bakhmutov,
V. I.; Howard, J. A. K.; Keen, D. A.; Kuzmina, L G.; Leech, M. A.;
Nikonov, G. I.; Vorontsov, E. V.; Wilson, C. C. J. Chem. Soc., Dalton
Trans. 2000, 1631. (c) Nikonov, G. I.; Kuzmina, L. G.; Howard, J. A. K.
J. Chem. Soc., Dalton Trans. 2002, 3037. (d) Dorogov, K.Yu.; Dumont,
E.; Ho, N.-N.; Churakov, A. V.; Kuzmina, L. G.; Poblet, J.-M.; Schultz,
A. J.; Howard, J. A. K.; Bau, R.; Lledos, A.; Nikonov, G. I. Organometallics
2004, 23, 2845. (e) Dorogov, K.Yu.; Yousufuddin, M.; Ho, N.-N.; Churakov,
A. V.; Kuzmina, L. G.; Schultz, A. J.; Mason, S. A.; Howard, J. A. K.;
Lemenovskii, D. A.; Bau, R.; Nikonov, G. I. Inorg. Chem. 2007, 46, 147.
† Nizhny Novgorod State University.
‡ University of Oxford.
§ Moscow State University.
Brock University.
(1) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. 1999, 38, 428–447. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283.
(2) (a) Gavenonis, J.; Tilley, T. D. Organometallics 2004, 23, 31. (b)
Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549. (c) Burckhardt,
U.; Casty, G. L.; Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21,
3108. (d) Castillo, I.; Tilley, T. D. J. Organomet. Chem. 2002, 643-644,
431. (e) Casty, G. L.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1997, 16, 4746. (f) Burckhardt, U.; Tilley, T. D. J. Am.
Chem. Soc. 1999, 121, 6328. (g) Wu, Z.; Xue, Z. Organometallics 2000,
19, 4191. (h) McAlexander, L. H.; Hung, M.; Li, L.; Diminnie, J. B.; Xue,
Z.; Yap, G. P. A.; Rheingold, A. L. Organometallics 1996, 15, 5231. (i)
Chen, T.; Sorasaenne, K. R.; Wu, Z.; Diminnie, J. B.; Xue, Z. Inorg. Chim.
Acta 2003, 345, 113. (k) Gavenonis, J.; Tilley, T. D. Inorg. Chem. 2004,
14, 4353. (l) Xue, Z.; Li, L.; Hoyt, L. K.; Diminnie, J. B.; Pollitte, J. L.
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(3) Review: Yu, X.; Morton, L. A.; Xue, Z. Organometallics 2004, 23,
2210.
(4) For example: (a) Gade, L. H.; Mountford, P. Coord. Chem. ReV.
2001, 216-217, 65. (b) Muller, P.; Fruit, C. Chem. ReV. 2003, 103, 2905.
10.1021/om800553y CCC: $40.75
2008 American Chemical Society
Publication on Web 10/15/2008

Tantalum Silyl Hydrides
Organometallics, Vol. 27, No. 22, 2008 5969
Scheme 1. Reactions of Cp(ArN)M(PMe3)2 (M ) Nb, Ta) with Hydrosilanes
like ligand platforms. In particular, we are interested in studying
the effect of supporting ligand sets on the extent of interligand
hypervalent interactions (IHI) between silyl and hydride ligands,
which were originally discovered for group 58 and 49 metal-
locene complexes. We have previously established5 that Cp-
(ArN)Ta(PMe3)2 (1, Ar ) 2,6-diisopropylphenyl) reacts with
HSiClMe2 to afford the silylhydrido complex Cp(ArN)-
Ta(PMe3)(H)(SiClMe2) (2b), whereas the corresponding reaction
of Cp(ArN)Nb(PMe3)2 gives the ꢀ-agostic silylamido compound
Cp{η3-N(Ar)SiMe2-H}NbCl(PMe3) (Scheme 1).10 The silyl
hydride 2b, like its isolobal niobocene analogues
Cp2Nb(SiClMe2)(H)(X) (X ) H, SiMe2Cl),11 was shown to have
IHI according to X-ray analysis and DFT calculations. By
applying more Lewis acidic silanes HSiCl2R (R ) Me, Cl), we
prepared metastable niobium complexes Cp(ArN)Nb(PMe3)-
(H)(SiCl2R), which rearrange/decompose upon phosphine ca-
talysis into silyl chlorides Cp(ArN)Nb(PMe3)(Cl)(SiClHR) and/
or to the hydride Cp(ArN)Nb(PMe3)(Cl)(H).12 Mechanistic
studies established that silane addition to Cp(RN)Nb(PMe3)2
goes via an unusual coupling of silane with the nitrogen center
rather than via the common Si-H bond oxidative addition to
the metal. We then set out to investigate the effect of varying
the ligand substituents on the extent of interligand interactions13
in the corresponding tantalum chemistry. Preliminary results14
for the complexes Cp(ArN)Ta(PMe3)(H)(SiXR2) revealed an
unusual dependence of silicon-hydrogen coupling constants
upon the substituents at silicon. Full details of this research and
insights into the mechanism of formation of the complexes
Cp(ArN)Ta(PMe3)(H)(SiR3) are reported herein.
Results and Discussion
1. Reactions of Cp(ArN)Ta(PMe3)2 (1) with Silanes. The
tantalum complex 1 (Ar ) 2,6-diisopropylphenyl) readily reacts
with a series of silanes HSiXR2 to give the silyl hydride
derivatives 2 (eq 1). In none of the reactions were agostic
compounds similar to the niobium complex Cp{η3-N(Ar)SiMe2-
H}NbCl(PMe3) (Scheme 1) observed. The products 2a-e were
characterized by spectroscopic methods (multinuclear NMR and
IR) and by X-ray diffraction studies of Cp(ArN)Ta-
(PMe3)(H)(SiClMe2)5a (2b) and Cp(ArN)Ta(PMe3)(H)(SiCl2Me)
(2d).14 In particular, the hydride signals appear as characteristic
doublets due to the coupling with the phosphine at rather low
field (about 5 ppm), apparently as a result of the anisotropy of
the M-N multiple bond.2a,15
(9) Ignatov, S. K.; Rees, N. H.; Tyrrell, B. R.; Dubberley, S. R.;
Razuvaev, A. G.; Mountford, P.; Nikonov, G. I. Chem.-Eur. J. 2004, 10,
4991.
(10) Several d0 silylamido agostic complexes are known:(a) Herrmann,
W. A.; Huberand, N. W.; Behm, J. Chem. Ber. 1992, 125, 1405. (b)
Procopio, L. J.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 1994, 116,
177. (c) Herrmann, W. A.; Eppinger, J.; Spiegler, M.; Runte, O.; Anwander,
R. Organometallics 1997, 16, 1813. (d) Nagl, I.; Scherer, W.; Tafipolsky,
M.; Anwander, R. Eur. J. Inorg. Chem. 1999, 1405. (e) Eppinger, J.;
Spiegler, M.; Hieringer, W.; Herrmann, W. A.; Anwander, R. J. Am. Chem.
Soc. 2000, 122, 3080.
(11) Cp/imido ligand set is isolobal with the Cp2 ancillary:(a) Poole,
A. D.; Gibson, V. C.; Clegg, W. J. Chem. Soc., Chem. Commun. 1992,
237. (b) Siemeling, U.; Gibson, V. C. J. Organomet. Chem. 1992, 426,
C25. (c) Cockcroft, J. K.; Gibson, V. C.; Howard, J. A. K.; Poole, A. D.;
Siemeling, U.; Wilson, C. J. Chem. Soc., Chem. Commun. 1992, 1668.
(12) Ignatov, S. K.; Rees, N. H.; Merkoulov, A. A.; Dubberley, S. R.;
Razuvaev, A. G.; Mountford, P.; Nikonov, G. I. Chem.-Eur. J. 2008, 14,
296–310.
Among the different silanes used in eq 1, chlorosilanes react
with 1 within a few minutes, whereas the reaction with
H2SiMePh was complete only after 14 h, the products being
two isomers of 2a (ratio 7:1).16 Heating an NMR sample of
this mixture in C6D6 at 60 °C for 1 h results in a rearrangement,
changing the ratio to 3:2 without decomposition. Further heating
at 110 °C for 5 h does not change the composition of the
(13) Recent reviews on nonclassical Si-H interactions: (a) Kubas, G. J.
Metal Dihydrogen and σ-Bond Complexes; Kluwer Academic/Plenum: New
York, 2001. (b) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32,
789. (c) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175. (d)
Lin, Z. Chem. Soc. ReV. 2002, 31, 239. (e) Nikonov, G. I. AdV. Organomet.
Chem. 2005, 53, 217.
(15) Green, M.L. H.; Konidaris, P. C.; Mountford, P. J. Chem. Soc.
Dalton. Trans. 1994, 2975. (b) Chernega, A. N.; Green, M. L. H.; Sua´rez,
A. G. J. Chem. Soc. Dalton. Trans. 1993, 3031.
(16) In fact, since 2a has two centers of chirality (tantalum and silicon),
it is more accurate to say that two diastereomeric pairs of enantiomers are
observed by 1H NMR.
(14) Dubberley, S. R.; Ignatov, S. K.; Rees, N. H.; Razuvaev, A. G.;
Mountford, P.; Nikonov, G. I. J. Am. Chem. Soc. 2003, 125, 642.

5970 Organometallics, Vol. 27, No. 22, 2008
Scheme 2. Decomposition Pathway for Complexes 2d and 2e in Mother Liquor
IgnatoV et al.
mixture, thus establishing the presence of equilibrium.17 An
NMR tube reaction of 1 with HSiPh2Cl showed an intractable
mixture of products; thus a preparative scale reaction was not
pursued. No reaction occurs with the bulkier silane HSiiPr2Cl
over the course of several weeks.
decomposition was achieved after heating the reaction mix-
ture at 110 °C for 2 h. In contrast, when 2d is heated in the
presence of 15 equiv of PMe3, the reaction occurs at a noticeable
rate already at 50 °C and in 2.5 h the rearranged product 5d is
formed in 50% yield, the second major product being 3. Further
heating at this temperature results in a clean transformation of
5d into 3. Moreover, with 9 equiv of PMe3 added, the compound
2d rearranges eVen at room temperature over the course of
several days into a mixture of 5d, 3, and 4, achieving a 1:0.54:
0.20 ratio after 3 days, with the starting 2d and other minor
coproducts contributing to about 35% of the intensity in the
Cp region. In the absence of phosphine, pure 2d is stable in
solutions at least for several weeks.
Complexes 2a-e are stable in the solid state and in solutions
containing the pure complex. However, keeping the chlorosilyl
complexes 2b-e in the mother liquor leads to decomposition
(Scheme 2). Thus, monitoring the reaction of 1 with HSiCl3 at
room temperature by NMR in C6D6 reveals, in addition to 2e,
a Very slow formation of two more products, Cp(ArN)Ta
(PMe3)Cl2 (3, Ar ) 2,6-diisopropylphenyl) and a new hydri-
dochloride derivative, Cp(ArN)Ta(PMe3)(H)Cl (4).18 Complex
4 exhibits a low-field hydride signal at 10.26 coupled to
phosphorus with J(P-H)) 69.1 Hz, the large value of coupling
constant being indicative of a cis arrangement of the hydride
and phosphine ligands. Another intermediate, having a hydride
signal at 7.91 (d, J(P-H)) 5.6 Hz) and a Cp signal at 5.86 (d,
J(P-H) ) 1.8 Hz), is tentatively assigned to the rearranged
structure Cp(ArN)Ta(PMe3)(Cl)(SiHCl2) (5e).19 A similar rear-
rangement of complexes Cp(ArN)Nb(PMe3)(H)(SiCl2R) to
Cp(ArN)Nb(PMe3)(Cl)(SiHClR) has been found in related
niobium chemistry.5c,12 Heating a sample of pure 2e in C6D6
in a sealed NMR tube results in the formation of 3 after 12 h
at 80 °C. However, in the presence of 3 equiv of PMe3 a 40%
decomposition of 2e occurred within 2 h at 80 °C, the major
products being 3 and the rearranged complex 5, formed in a
4:3 ratio. In the presence of 3 equiv of PMe3, complete
transformation of 2e to 3 at room temperature requires several
days.
In contrast, thermal decomposition of Cp(ArN)Ta(PMe3)-
(H)(SiMe2Cl) (2b) is slow in both the presence and absence of
PMe3. A significant reaction is observed only at temperatures
above 70 °C and requires 1.5 days to reach 50% conversion.
Among various Cp and silane products formed, only H2SiMe2
1
was identified (by its characteristic H NMR signals). Finally
we note that attempted reactions of Cp(ArN)Ta(PMe2Ph)2 with
HSiMe2Cl and HSiMeCl2 gave Cp(ArN)Ta(PMe2Ph)Cl2 as the
only tantalum-containing product.
2. Reactions of Cp(Ar′N)Ta(PMe3)2 with Silanes. The
compound Cp(Ar′N)Ta(PMe3)2 (6, Ar′ ) 2,6-dimethylphenyl),
featuring a less bulky 2,6-dimethylphenyl substituent at nitrogen,
is analogous to 1 and exhibits similar reactivity toward silanes
HSiClR2 (eq 2). The only difference is that in this case the
reaction with HSiClPh2 does give high yields of the derivative
Cp(Ar′N)Ta(PMe3)(H)(SiClPh2) (7f). The attempted reaction
with HSiiPr2Cl does not afford an isolable silyl or agostic
derivative, but instead slowly produces a complex reaction
mixture of yet unidentified products over a period of several
days. The compounds 7 were characterized by NMR and IR
spectroscopy and by an X-ray diffraction study of 7d.
The compound 2d slowly rearranges to 5d in the mother
liquor at room temperature so that after 2 days 35% conversion
is achieved, with the yield of 5d amounting to 22%. For
comparison, the analogous rearrangement of Cp(ArN)Nb-
(PMe3)(H)(SiCl2Me) takes several hours. Heating an NMR
sample of pure 2d at 50 °C for 21 h does not result in any
significant decomposition, whereas an increase in temperature
to 60 °C leads to a noticeable reaction, so that after 1.5 h the
rearranged product 5d contributes to about 30% intensity of
1
the Cp-containing products in H NMR. However, this trans-
formation is not clean and several other coproducts are formed.
One of them was identified as Cp(ArN)Ta(PMe3)(H)(Cl) (4)
(vide supra), a likely product of net silylene (:SiClMe) extrusion
from 5d. Further increasing the temperature to 90 °C results in
decomposition to Cp(ArN)Ta(PMe3)Cl2 (3) after 2.5 h. Complete
The molecular structure of 7d is shown in Figure 1. This
compound is analogous to the previously reported complex 2d14
and exhibits very similar molecular parameters (e.g., the Ta-Si
bond is 2.569(2) Å in 2d vs 2.570(6) Å in 7d). Complex 7d
appears to have two different Si-Cl bond lengths (2.116(7) Å
to the chloride lying trans to hydride and 2.109(9) Å to the cis
Cl), although due to the relatively large esd’s, the difference is
not statistically significant. In the closely related compound 2d
two types of Si-Cl distances were observed, which was
(17) The composition of the mixture was established by NMR experi-
ments that were run at room temperature after the thermal experiments.
Since the ratio of two diastereomers does not change with temperature, it
suggests that their interconversion occurs with a relatively small barrier.
No high-temperature NMR experiments were attempted.
(18) This compound has been prepared by an independent method and
fully characterized. G. I. Nikonov, unpublished.
(19) We could not confirm this structure by the 29Si NMR due to the
low content of this species in the sample, but its niobium analogue has
been previously described (ref 12).

Tantalum Silyl Hydrides
Organometallics, Vol. 27, No. 22, 2008 5971
in the more chlorinated members of this series.14 Specifically,
introduction of a chlorine group on silicon on going from 2a to
2b leads to the “switching on” of the interligand hypervalent
interaction,14b but further chloride substitution at silicon results
in the weakening of the Si-H bonding. Such behavior appears
to be mirrored in the variation of the hydride signals in the 1H
NMR spectra (Table 1), which shift to a higher field from 2a
to 2b but progressively move to a lower field from 2b to 2e.21
An analogous “V”-type dependence on the number of Cl groups
on Si was also observed for the Ta-H stretches in the IR spectra
(Table 1). The high-field shift of the hydride resonance in the
1H NMR spectra and the reduced frequency of the M-H band
in the IR spectra serve as independent (although still indirect)
indicators of the participation of the tantalum-bound hydride
in a nonclassical bonding with the silicon atom ligand.13e
It is interesting to compare this result with silane σ-complexes,
in which electron-withdrawing groups at silicon tend to promote
more advanced Si-H addition (i.e., less residual Si-H interac-
tion) to the metal,13 although relatively large values of J(Si-H)
can be still observed.20 In contrast, chloride substitution in the
agostic complex (Ar′N)(η3-Ar′NSiMe2-H · · · )Mo(Cl)(PMe3)2
(Ar′ ) 2,6-dimethylphenyl) to give the analogue (Ar′N)(η3-
Ar′NSiMeCl-H · · · )Mo(Cl)(PMe3)2 results in slight strengthening
of the Si-H interaction, although the value of J(Si-H) increases
quite substantially (from 97 to 129 Hz).5c In all three cases (IHI,
silane σ-complexes, and silylamido ꢀ-agostic complexes) the
origin for the abnormal behavior of silicon-hydride coupling
constants is the same: it stems from the relative increase of Si
3s character in the Si-H bond, caused by the chlorine
substitution at silicon.13e
Figure 1. Molecular structure of 7d. Hydrogen atoms are omitted
for clarity. The hydride atom has not been determined. Selected
bond distances (Å) and bond angles (deg): Ta1-P1 2.542(5),
Ta1-Si1 2.570(6), Ta1-N1 1.794(14), Si1-Cl1 2.116(7), Si1-Cl2
2.109(9), Si1-C17 1.84(2), P1-Ta1-Si1 127.54(18), P1-Ta1-N1
90.7(4),Si1-Ta1-N197.7(4),Ta1-Si1-Cl1114.3(3),Ta1-Si1-Cl2
110.8(3), Cl1-Si1-Cl2 101.9(4), Ta1-Si1-C17 121.6(11),
Cl1-Si1-C17 103.4(12), Cl2-Si1-C17 102.6(10), Ta1-N1-C6
173.2(12).
attributed to the involvement of the trans Si-Cl bond in IHI
with the Ta-H bond.8-14
We have previously argued that the sign of the coupling
constant, rather than its absolute value, serves as a more reliable
signature of the presence of nonclassical bonding.9 Namely, a
negative sign of J(Si-H) indicates the presence of direct Si-H
bonding due to the fact that silicon has a negative gyromagnetic
ratio.22 Indeed, negative values for J(Si-H) were measured or
calculated for complexes for which the presence of interligand
Si-H interactions was independently invoked from X-ray and/
or DFT data.9,23 We have now succeeded in the experimental
determination of the sign of J(Si-H) in some complexes of
type 2 (Table 1). In the compound Cp(ArN)Ta(PMe3)-
(H)(SiMePhH) (2a, Ar ) 2,6-diisopropylphenyl) the J(Si-H)
was found to be positive, consistent with its classical silyl-hydride
formulation. In contrast, in the chloro-substituted complexes
Cp(ArN)Ta(PMe3)(H)(SiMeCl2) (2d) and Cp(ArN)Ta-
(PMe3)(H)(SiCl3) (2e) the signs of the Si-H coupling constants
are negatiVe, thus providing ultimate experimental confirmation
of the presence of significant interligand Si-H interactions, in
accord with the results of our X-ray studies and DFT calculations
(vide infra).
Heating an NMR sample of pure 7b in C6D6 at 60 °C for
several hours produces a complex reaction mixture containing
the silanes HSiMe2Cl and H2SiMe2 among other, as yet
uncharacterized, products. No signals attributable to the putative
agostic compound Cp(Ar′NSiMe2-H)TaCl(PMe3) or the rear-
ranged product Cp(Ar′N)Ta(PMe3)(Cl)(SiMe2H) were observed
in the 1H NMR spectrum. Thermolysis of 7d in C6D6 at 60 °C
for several hours also gives a mixture of unidentified products.
By way of contrast, heating 7c (mixture of two isomers) in C6D6
at 65 °C in a sealed NMR tube does produce the rearranged
product Cp(Ar′N)Ta(PMe3)(Cl)(SiMePhH) (8, two isomers)
identified by the characteristic pattern for the SiMePhH group.
Due to the complexity of the reaction mixture and the presence
of decomposition products, a preparative scale reaction was not
pursued.
3. Spectroscopic Studies of Complexes Cp(RN)Ta(PMe3)-
(H)(SiMenCl3-n) (R ) Ar, Ar′). The silyl hydride complexes 2
(Ar ) 2,6-diisopropylphenyl) and 7 (Ar′ ) 2,6-dimethylphenyl)
show an unusual trend in that the absolute values of the
experimentally determined J(P-H) and J(Si-H) constants
increase (by up to 50 Hz for the J(Si-H) in 2e) upon increasing
electronegativity of the substituents at Si (Table 1). This
observation is of interest in view of the fact that most of the
previously reported complexes with nonclassical Si-H interac-
tions were characterized by J(Si-H) values larger than 20 Hz,
with the coupling constant decreasing with increasing elec-
tronegativity of substituents at Si.13,20 Surprisingly enough, our
preliminary DFT and X-ray data for complexes 2 showed that
larger J(Si-H) values correspond to weaker Si-H interactions
4. Mechanism of Silane Addition. Our previous kinetic and
DFT study of the reactions of Cp(ArN)Nb(PR′3)2 (Ar ) 2,6-
diisopropylphenyl) with silanes established the possibility of a
(21) The chemical shift of monochloro-substituted complex 2c (5.82
and 5.68 ppm for two isomers) are downfield shifted relative to the chlorine
free complex 2a. However, given the fact that the phenyl group is a π-donor,
which may result in descreased IHI, the origin of this shift is difficult to
rationalize unequivocally.
(22) A negative J(Si-H) of -201.3 Hz was measured in SiH4: (a)
Jackowski, K. Int. J. Mol. Sci. 2003, 4, 135. (b) Sauer, S. P. A.; Raynes,
W. T.; Nicholls, R. A. J. Chem. Phys. 2001, 115, 5994.
(20) See, however, an exception of this trend: (a) Nikonov, G. I.
Organometallics 2003, 22, 1597. (b) Lichtenberger, D. L. Organometallics
2003, 22, 1599. (c) Bader, R. F. W.; Matta, C. F.; Corte´s-Guzma´n, F.
Organometallics 2004, 23, 66253. (d) Also see the comment on page 227
of ref 13e for the summary of the debate.
(23) Osipov, A. L.; Vyboishchikov, S. F.; Dorogov, K. Y; Kuzmina,
L. G.; Howard, J. A. K.; Lemenovskii, D. A.; Nikonov, G. I. Chem.
Commun. 2005, 3349. (b) Vyboishchikov, S. F.; Nikonov, G. I. Chem.-
Eur. J. 2006, 12, 8518. (c) Vyboishchikov, S. F.; Nikonov, G. I.
Organometallics 2007, 26, 4160.

5972 Organometallics, Vol. 27, No. 22, 2008
IgnatoV et al.
Table 1. Selected NMR (in C6D6) and IR (in Nujol) Parameters of Complexes 2 and 7
compound
1H, δ
J(P-H), Hz
29Si, δ
J(P-Si), Hz
J(Si-H), Hz
ν(Ta-H), cm-1
2a
2b
5.68 (dd, J(H-H) ) 4.7 Hz)a
63.5
18.8
8.0
-179 (terminal Si-H)
+ 14 (Si-H-Ta)
|33.3|b
1674
5.13 (d)
5.82
5.68
6.14 (d)
6.74 (d)
5.03 (d)
5.60
5.65 (d)
6.00 (d)
6.32 (d)
64.2
64.5
64.5
67.2
70.5
61.5
55.0
63.1
64.8
63.3
91.7
87.4
87.0
104.9
83.5
91.6
87.4
90.0
106.1
85.5
15.0
14.6
12.2
15
1650c
1660
2c (two isomers)
-41
2d
2e
7b
-41.5
-50
1660
1668
1650
1652
14
d
|30|b
d
7c (two isomers)
7d
7f
15
11.4
-41
1650
1676
a An erroneous value of 5.87 ppm was given in ref 14. The SiH signal is observed at 6.18 ppm corresponding to the IR stretch at 2062 cm-1 b The
.
sign cannot be determined because of insufficient separation of the 29Si satellites from the main signal. c An erroneous value of 1736 cm-1 is given in
the earlier paper (ref 5). d The value of J(Si-P) is absent because the 29Si NMR was not recorded. The silicon signal was determined from a 1H-29Si
HMQC; the J(Si-H) was measured from silicon satellites in the 1H NMR spectrum.
Figure 2. Relative free energies of the products in the reaction between complex 10 and HSiMe2Cl (in kcal/mol).
new mechanism of silane activation.12 This pathway includes
direct addition of silanes to the imido moiety to give an
intermediate structure, Cp(ArNfSiR2Cl-H · · · )Nb(PR′3)2 (9),
with a pentacoordinate silicon center and an additional agostic
Si-H · · · Nb interaction. The Si-H activation products
Cp(ArN)Nb(PR′3)(H)(SiR2Cl) were found to be kinetic products
of silane addition. However, in the presence of phosphine, they
rearrange, likely via the same intermediate 9, into the products
of Si-Cl bond activation, such as complexes Cp(ArN-SiR2-
H · · · )Nb(PR′3)(Cl) (R2 ) Me2, MePh) and Cp(ArN)Nb-
(PMe3)(Cl)(SiR2H) (R2 ) MeCl, Cl2).12
A very similar scenario appears to be happening for the
tantalum complexes Cp(RN)Ta(PMe3)2 (R ) Ar, Ar′). The
initially formed products Cp(ArN)Ta(PMe3)(H)(SiXR2) are
metastable and decompose in the presence of phosphine into
Cp(ArN)Ta(PMe3)(Cl)(SiHR2), Cp(ArN)Ta(PMe3)(Cl)(H), and
eventually Cp(ArN)Ta(PMe3)Cl2, but the rate of this decom-
position is noticeably slower than for niobium analogues. Like
for the Nb complexes, the presence of phosphine accelerates
the reactions significantly, suggesting that a diphosphine
intermediate akin to 9 is involved. But in contrast to the Nb
chemistry, in none of the reactions was an agostic complex of
tantalum, such as Cp(ArN-SiR2-H · · · )Ta(PMe3)(Cl), observed.
To understand better the factors controlling the outcome of
these very complex transformations, we carried out DFT
calculations on model complexes 10-19 (Figures 2-4, Tables
2, 3). For computational simplicity we used a methyl substituent
at nitrogen. However, exact modeling of substituents at phos-
phorus and silicon was applied because our previous studies
showed that this was important.5a A good agreement between
the calculated and experimental geometries is observed, when
the latter are available from X-ray analyses.
In accord with our experimental findings, the most thermo-
dynamically stable product of HSiClMe2 addition to the
diphosphine complex 10 is the silyl hydride 11trans (Figure
2). A rotamer of 11trans, the complex 11cis, having the chloride
substituent at Si in the cis position to the hydride is 3.0 kcal/
mol less stable (free energy scale), due to the loss of Si-H
interligand hypervalent interactions (IHI). This is seen from a
noticeable elongation of the Ta-Si bond from 2.583 Å in
11trans to 2.594 Å in 11cis, accompanied by a shortening of
the Si-Cl bond from 2.171 Å to 2.148 Å and the contraction
of the Ta-H bond from 1.812 Å to 1.798 Å.24
A structural isomer of 11, the silyl chloride complex
Cp(MeN)Ta(PMe3)(Cl)(SiHMe2) (12), is further destabilized by
1.7 kcal/mol (Figure 2). A structure of this type is possibly
formed at high temperature in the course of thermal decomposi-
tion of compounds 2 to the dichloride 3. The agostic compound
13 is the least stable product of silane addition. Although it is
thermodynamically feasible (∆Gr ) -1.7 kcal/mol), it lies 9.0
kcal/mol (free energy scale) above 11trans, thus accounting
for our inability to observe such a structure experimentally. As
(24) Structurally, IHI is characterized by elongated Si-X bonds,
elongated M-H bonds, shortened M-Si bonds, and shortened Si-H
contacts. The presence of an electron-withdrawing group X trans to the
metal-bound hydride is a prerequisite of IHI, well documented in previous
structural and theoretical studies (refs 5a, 5b, 8, 9, 12, and 14).

Tantalum Silyl Hydrides
Organometallics, Vol. 27, No. 22, 2008 5973
Figure 3. Possible reaction pathways in the reaction between complex 10 and HSiMe2Cl. Free energies (in kcal/mol) are taken relative to
11trans.
Figure 4. Relative free energies of stable intermediates in the preparation of 17 and 18.

5974 Organometallics, Vol. 27, No. 22, 2008
IgnatoV et al.
Table 2. Selected Interatomic Distances (Å) of the Ta Structures 11-13 and 15
structures
13
11cis
11trans
12
15cis
1.924
15trans
Ta-H
Ta-N
Ta-P4, P5
Ta-Si
Ta-Cl
Si-N
1.798
1.789
2.527
2.594
3.879
3.116
2.243
2.148
1.812
1.788
2.533
2.583
3.920
3.166
2.174
2.171
2.008
2.020
2.560
2.721
2.496
1.728
1.570
3.869
1.805
2.062
2.523,2.523
3.199
5.080
1.753
2.223
2.174
1.782
2.633
2.661
2.538
1.987
2.557,2.536
2.910
4.439
1.805
Si-H
Si-Cl
1.502
1.635
2.410
Table 3. Selected Interatomic Distances (Å) of the Ta Structures 10,
stable structure is obtained (27.5 vs 34.2 kcal/mol for 15cis),
but the required barrier TS10-15trans is prohibitively high (48.4
kcal/mol) (Figure 3).
14, 17-19
10
1.802
14
16
1.903
1.793 1.991
17
1.808
1.786
18
19
Ta-H
Ta-N
2.156
In contrast to the agostic complex 15cis, 15trans is essentially
a silylamido hydride derivative characterized by a shorter Si-N
bond length (1.753 vs 1.805 Å in 15cis), a longer Ta-N bond
(2.062 vs 1.987 Å in 15cis), a shorter Ta-H bond (1.805 vs
1.924 Å in 15cis), and a significantly longer Si-H distance
(2.223 vs 1.635 Å in 15cis). The corresponding transition state
TS10-15trans features the silicon atom in a distorted trigonal
-bipyramidal geometry with an unusual trans disposition of the
methyl groups.26 The Si-Cl bond is significantly elongated to
2.509 Å, whereas the Si-N distance is shortened to 1.875 Å.
The Si-H bond is stretched to 1.603 Å, whereas the incipient
Ta-H bond is still rather long (2.045 Å). The occupation of
apical sites by electron-donating methyl groups and the stretch-
ing of Si-Cl and Si-H bonds suggests a possible origin of
such a high barrier.
In the case of the more Lewis acidic dichlorosilane HSiCl2Me,
the enthalpy of its addition to 10 to give the initial adduct 16 is
negative (-1.2 kcal/mol), but 16 is still less stable by 16.8 kcal/
mol on the free energy scale than the starting diphosphine
complex 10 due to the loss of entropy (Figure 4). Complex 16
is further stabilized relative to the unsaturated complex Cp-
(MeN)Ta(PMe3) (14) in comparison with the related adduct
15cis in the case of HSiMe2Cl addition (by 10.7 vs 4.0 kcal/
mol, respectively). This extra stabilization can be traced to
shorter Si-N and Si-Ta bonds in 16 (Tables 2 and 3), but
other structural features are very similar in both compounds.
The formation of the silyl hydride complex 17 from 10 is
thermodynamically allowed (∆Gr ) -16.8 kcal/mol), but the
most stable product is the “rearranged” chloride silyl derivative
Cp(MeN)Ta(PMe3)(Cl)(SiHClMe) (18) (Figure 4). This result
agrees well with the observed phosphine-induced decomposi-
tions of the real silyl hydride complexes 2d and 2e to
Cp(ArN)Ta(PMe3)(Cl)(SiHClX) (X ) Me or Cl), Cp(ArN)-
Ta(PMe3)(Cl)(H), and Cp(ArN)Ta(PMe3)Cl2. Interestingly, the
formation of the agostic complex 19 from 10 is in fact
thermodynamically unfavorable (Figure 4).
1.780 2.016
2.625 2.531
2.629 2.758
2.541 2.492
1.723
Ta-P4, P5 2.500, 2.490 2.507 2.532, 2.556 2.531
Ta-Si
Ta-Cl
Si-N
Si-H
Si-Cl
2.900
2.562
1.779
1. 635
2.197
1.497 1.512
2.286, 2.146 2.144,2.124 2.155 2.101
mentioned above, in the analogous niobium chemistry the Nb(V)
silyl hydride complex was a kinetic product of silane addition,
whereas the Nb(III) silylamido agostic complex was the
thermodynamic product. Structural parameters of the Ta com-
plexes 11-13 (Table 2) are very close to those of their Nb
analogues,12 which does not suggest an obvious reason why
the agostic complex 13 is so much destabilized relative to other
products. Taking into account that lower oxidation states become
less stable down the group, one can expect destabilization of
the Ta(III) agostic d2 structure relative to the Ta(V) silylhydrido
d0 form. Another possible factor could be the increased stability
of the metal-ligand multiple bonds down the group, which tend
to retain the TadNR functionality in the product.
Supporting this idea is the observation that Ta(V) complexes
11 and 12 are much more stable relative to the Ta(III) complex
10 than their Nb analogues relative to Cp(MeN)Nb(PMe3)2:
∆Gr(11) ) -10.7 kcal/mol versus ∆Gr ) -3.7 kcal/mol for
Cp(MeN)Nb(SiMe2Cl)(H)(PMe3) and ∆Gr(12) ) -6.0 kcal/
mol versus ∆Gr
(SiMe2H)(Cl)(PMe3).
)
2.1 kcal/mol for Cp(MeN)Nb-
As for the previously studied Nb complexes,12 we considered
two mechanisms of silane addition to the starting diphosphine
10. A dissociative mechanism goes via the transition state
TS10-1425 to afford a monophosphine intermediate 14 amenable
for Si-H bond activation at the metal center to afford
the complex 11 (Figure 3).
An alternative, associative pathway discussed here includes
silane addition to the metal-nitrogen multiple bond. We
considered two directions of such an attack. Addition of silane
in such an orientation that chloride is trans to nitrogen but cis
to the hydride gives the species 15cis, which features a
pentacoordinate silicon center and a Si-H · · · M agostic bonding
(the Si-H bond is 1.635 Å, the Ta-H bond is 1.924 Å). This
addition requires overcoming a barrier of 34.2 kcal/mol
(TS10-15cis), which is much below the barrier of phosphine
dissociation from 10 (49.3 kcal/mol). Another adduct, 15trans,
emerges when silane adds to 10 with the chlorine lying trans to
the hydride and cis to the nitrogen center. In this case, a more
In the related niobium chemistry, kinetic studies suggested
that the rearrangement of initial silyl hydride products into
agostic or chloride silyl products goes via an Nb analogue of
complex 16.12 In this context, it is rewarding that our calcula-
tions predict a greater free energy gap between the tantalum
intermediate 16 and the kinetic product 17 (∆G ) 33.6 kcal/
mol) than the previously found difference for their niobium
analogues (∆G ) 26.0 kcal/mol).12 This is in accord with our
experimental observation that niobium derivatives rearrange
(25) The electronic potential surface around complex 14 is very shallow,
which does allow for the determination of a transition state for the
dissociative pathway by the standard methods. For the methodology of
estimation of the barrier of phosphine dissociation from 10, see the
Supporting Information.
(26) In hypervalent compounds, such as pentacoordinate silanes, the
apical sites are preferentially occupied by the most electron-withdrawing
groups. See: Corriu, R. J. P.; Young, J. C. Hypervalent Silicon Compounds.
In Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z.,
Eds.;Wiley: New York, 1989; pp 1242-1288.

Tantalum Silyl Hydrides
Organometallics, Vol. 27, No. 22, 2008 5975
qualitatively faster than their tantalum congeners, thus further
supporting our inference that rearrangement of the silyl hydride
species 2 into chloride silyl products goes via bis(phosphine)
structures similar to 15 and 16.
Experimental Section
All manipulations were carried out using conventional glovebox
and Schlenk techniques. Solvents were dried over sodium or sodium
benzophenone ketyl. NMR spectra were recorded on a Varian
Mercury-Vx (1H, 300 MHz; 13C, 75.4 MHz) and Unity-Plus (1H,
500 MHz; 13C, 125.7 MHz) spectrometers. IR spectra were obtained
as Nujol mulls with a FTIR Perkin-Elmer 1600 series spectrometer.
Silanes were obtained from Sigma-Aldrich and Lancaster and
distilled over CaH2. Starting complexes Cp(RN)TaCl2 and Cp(RN)
Ta(PMe3)2 (R ) Ar, Ar′) were prepared by literature methods (see
Supporting Information for details). Compound 2b was previously
reported.5a Syntheses and characterization of complexes 2a,d,e are
reported in the Supporting Information of the preliminary com-
munication to this paper.14
Like 11trans, the complex 17 exhibits interligand hypervalent
interactions between the silyl and hydride ligands, as evidenced
by comparing two types of Si-Cl bonds in 17.24 Thus, the
Si-Cl bond lying trans to the hydride is longer than the cis
Si-Cl bond(s): 2.144 vs 2.124 Å. An analogous effect was
observed in other complexes with IHI and in the X-ray structure
of 8d discussed above. IHI also results in the elongation of the
Ta-H bonds, and in comparison with 17, the presumably
stronger Si-H interaction in 11trans leads to a longer Ta-H
bond in the latter (1.812 Å in 11trans vs 1.808 Å in 17). The
calculated Si-H distance increases from 2.174 Å in 11trans
to 2.197 Å in 17, further signifying the weakening of IHI. These
observations are in accord with our earlier conclusion that IHI
is the strongest in monochloro-substituted derivatives, which
have the most electron-depleted and hence the longest
metal-hydride bond.
Cp(ArN)Ta(PMe3)(H)(SiMePhCl) (2c). A 0.15 mL amount of
HSiMePhCl (1.0 mmol) was added to 20 mL of a hexane solution
of Cp(ArN)Ta(PMe3)2 (0.243 g, 0.424 mmol). The mixture was
left for 2 days at room temperature. After that the purple solution
was filtered and dried, producing a red oil. The oil was dissolved
in 10 mL of hexane and cooled to -30 °C. A yellow deposit was
formed on the walls in the course of several days. The solution
was filtered and the residue was dried, giving 0.091 g of a yellow
powder. The second crop was obtained in an analogous way. NMR
spectra showed the formation of a mixture of two diastereomers of
2c. Analytically pure material was not obtained even after multiple
recrystallization. Total yield: 0.141 g (0.22 mmol, 51%). IR (Nujol):
Conclusions
The silyl hydrides Cp(ArN)Ta(PMe3)(H)(SiR3) (2, Ar ) 2,6-
diisopropylphenyl) and Cp(Ar′N)Ta(PMe3)(H)(SiR3) (7, Ar′ )
2,6-dimethylphenyl) were found to be intermediates in the
reactions between diphosphine complexes Cp(R′N)Ta-
(PMe3)2 (R′ ) Ar or Ar′) with silanes H-SiR3. In the presence
of PMe3 they sequentially rearrange/decompose into Cp(R′N)Ta-
(PMe3)(Cl)(SiHRnCl2-n) (5 for R′ ) Ar and 8 for R′ ) Ar′),
Cp(R′N)Ta(PMe3)(Cl)(H), and eventually Cp(R′N)Ta-
(PMe3)Cl2. As in related niobium chemistry, DFT calculations
suggest that complexes Cp(R′N)Ta(PMe3)(H)(SiR3) and
Cp(R′N)Ta(PMe3)(Cl)(SiHRnCl2-n) are formed from the same
key intermediate, the imido/silane adduct Cp(R′Nf-
SiR3)Ta(PMe3)2 (15 for the model complex with R′ ) Me, R3
) Me2Cl; 16 for R′ ) Me, R3 ) MeCl2), as a result of Si-H
and Si-Cl bond activation, respectively. We studied two ways
how this key intermediate could be formed and found that
although addition of a chlorosilane with its Si-Cl bond oriented
cis to the imido group gives a more stable product, its reaction
barrier is energetically too demanding. Silane addition with the
Si-Cl bond trans to imido group is kinetically preferred and
1
νTa-H ) 1660 cm-1. H NMR (C6D6): δ 8.23 (d, J(H-H) ) 7.5
Hz, m-Ar), 8.09 (d, J ) 8.1 Hz, m-Ar), 7.33 (t, J ) 7.5 Hz, p-Ar),
7.27 (t, J ) 7.7 Hz, p-Ph), 7.21-6.93 (m, Ph), 5.78 (d, J(P-H))
22.2 Hz, 1, Ta-H), 5.61 (d, J ) 1.5 Hz, 5, Cp), 5.34 (d, J(P-H))
36.9 Hz, 1, Ta-H), 5.33 (d, J ) 1.5 Hz, 5, Cp), 4.08 (q, J(H-H)
) 6.7 Hz, 1, CHMe2), 3.86 (q, J(H-H) ) 6.8 Hz, 1, CHMe2),
1.40 (s, SiMe), 1.10 (s, SiMe), 0.93 (d, J(P-H) ) 8.7 Hz, 9, PMe),
0.92 (d, J(P-H) ) 8.7 Hz, 9, PMe). 13C{1H} NMR (C6D6): δ 143.1
(o-Ar), 142.9 (o-Ar), 135.1 (m-Ar), 134.2 (m-Ar), 127.8 (p-Ar),
127.6 (p-Ar), 127.5 (p-Ar), 122.9 and 122.7 and 119.9, 100.9 (Cp),
100.7 (Cp), 27.2 (CHMe2), 27.1 (CHMe2), 24.6 (CHMe2), 24.3
(CHMe2), 24.2 (CHMe2), 22.5 (CHMe2), 19.8 (d, J(P-C) ) 20.5
Hz, PMe), 19.6 (d, J(P-C) ) 19.5 Hz, PMe), 15.5 (s, SiMe), 14.1
5 (s, SiMe). 31P{1H} NMR (C6D6): δ -2.0. 29Si NMR (C6D6): δ
88.3 (2J(Si-P) ) 15.9 Hz, J(Si-P) ) 12.8 Hz).
Thermal Decomposition of Cp(ArN)Ta(PMe3)(H)(SiMeCl2)
(2d). An NMR sample of 2d was heated at 60 °C for 1.5 h to give
a
mixture of Cp(ArN)Ta(PMe3)(Cl)(SiClHMe) (5d, 30%),
affords
a
pentacoordinate silicon structure Cp(R′Nf
Cp(ArN)Ta(PMe3)(H)(Cl) (4), and other yet unidentified products.
Further increase of the temperature to 90 °C resulted in decomposi-
tion into Cp(ArN)Ta(PMe3)Cl2 (3) after 2.5 h. The complete
decomposition was achieved after heating the reaction mixture at
110 °C for 2 h.
SiR2Cl-H · · · )Ta(PMe3)2 supported by an additional agostic
Si-H · · · Ta interaction.
An important advance in our investigation of interligand
hypervalent interactions in Cp/imido complexes has been
achieved in the experimental determination of the sign of the
coupling constants between the hydride and silyl ligands in
complexes Cp(ArN)Ta(PMe3)(H)(SiR3) (2). The observed
J(Si-H) was found to be positive for the classical compound
Cp(ArN)Ta(PMe3)(H)(SiMeHPh) (2a) and negative for the
1
5d. IR (Nujol): νTa-H ) 2122 cm-1. H NMR (C6D6): δ 7.06
(d, J(H-H) ) 8.0 Hz, 2, m-Ar), 6.87 (pt, J(H-H) ) 8.0 Hz, 1,
p-Ar), 6.75 (dq, J(H-H) ) 4.0 Hz, 1, SiH), 5.72 (s, 5, Cp), 3.80
(sept, J(H-H) ) 7.0 Hz, 1, CHMe2), 1.33 (d, J(H-H) ) 7.5 Hz,
6, CHMe2), 1.26 (br, 3, SiMe), 1.15 (d, J(H-H) ) 7.0 Hz, 6,
CHMe2), 0.91 (d, J(P-H) ) 8.7 Hz, 9, PMe). The position of the
SiMe group was determined by a COSY experiment, which showed
that the hydride on Si at 6.75 ppm is coupled to the broad signal
at 1.26 ppm. 31P{1H} NMR (C6D6): δ -10.7 ppm. 29Si HMQC: δ
54.3 ppm; this signal is coupled to the hydride at 6.75 ppm and
the methyl group at 1.26 ppm.
3. This compound was independently prepared by a literature
procedure described for its Nb analogue.7c 1H NMR (C6D6): δ 7.07
(d, J(H-H) ) 7.5 Hz, 2, m-Ar), 6.87 (pt, J(H-H) ) 7.7 Hz, 1,
p-Ar), 5.95 (s, 5, Cp), 3.82 (sept, J(H-H) ) 6.9 Hz, 1, CHMe2),
1.32 (d, J(H-H) ) 6.9 Hz, 6, CHMe2), 1.27 (d, J(H-H) ) 6.9
Hz, 6, CHMe2). 31P{1H} NMR (C6D6): δ -1.3 ppm. Anal. Calcd
chloro-substituted complexes Cp(ArN)Ta(PMe3)(H)(SiRnCl3-n
)
(2b-e, n ) 1-3). The latter fact conclusively proves the
existence of direct Si-H interactions. Interestingly, the Si-H
bonding diminishes when the absolute value of the J(Si-H) in
Cp(ArN)Ta(PMe3)(H)(SiRnCl3-n) (n ) 1-3) increases. This
trend contradicts the previously accepted view that hydride-silyl
coupling constants should decrease when the strength of
nonclassical bonding decreases. Recent studies indicate that this
“abnormal behavior” of J(Si-H) is general and can be found
also in silane σ-complexes and agostic complexes.5c,20a Its
rationalization in terms of Bent’s rule effects and rehybridization
of the silicon center can be found in a recent review.13e

5976 Organometallics, Vol. 27, No. 22, 2008
IgnatoV et al.
Table 4. X-ray Diffraction Crystal Data and Structure Refinement
for 7d
for C20H31Cl2TaP (568.294): C, 42.27; H, 5.50, N 2.46. Found: C,
42.43; H, 5.57; N, 2.19
Cp(Ar′N)Ta(PMe3)(H)(SiMe2Cl) (7b). To a suspension of
Cp(NAr′)Ta(PMe3)2 (0.22 g, 0.425 mmol) in a 2:1 mixture of ether/
pentane (50 mL) was added 0.2 mL (1.7 mmol) of HSiMe2Cl to
give after keeping overnight a yellow solution and a small amount
of gray precipitate. The solution was filtered and cooled to -80
°C to produce yellow crystals. The crystals were filtered and dried.
formula
fw
C17H27Cl2NPSiTa
556.31
brown-red, block
0.24 × 0.24 × 0.40
monoclinic
P21/n
11.3952(3)
14.8865(4)
13.2074(5)
106.379(1)
2149.51(12)
4
150
1.716
1088.000
Mo
54.91
0.14 -0.27
54.97
8483
4781
0.02
2326 (n ) 2)
209
color, habit
cryst size, mm
cryst sys
space group
a, Å
Yield: 0.05 g (0.093 mmol, 22%). IR (Nujol): νTa-H ) 1650 cm-1
.
b, Å
c, Å
1H NMR (C6D6): δ 7.02 (d, J(H-H) ) 7.2 Hz, 2 H, m-C6H3),
6.81 (pt, J ) 7.8 Hz, 1 H, p-C6H3), 5.48 (d, J(P-H) ) 1.8 Hz, 5
H, Cp), 5.03 (d, J(P-H) ) 61.5 Hz, 1 H, Ta-H), 2.35 (s, 6 H,
C6H3Me2), 1.22 (s, 3, SiMe), 0.97 (s, 3, SiMe), 0.84 (d, J(P-H))
8.7 Hz, 9 H, PMe). 13C{1H} NMR (C6D6): δ 156.5, 132.5, 127.8,
121.67 (all Ar′), 100.2 (Cp), 21.3 (C6H3Me2), 20.3 (SiMe), 19.9
(SiMe), 15.5 (bs, PMe). 31P{1H} NMR (C6D6): δ -2.6 (s). 29Si
NMR (C6D6): δ 91.6 (J(Si-H) ) 30 Hz). Anal. Calcd for
C18H30ClNTaPSi (535.905): C, 40.34; H, 5.64, N 2.61. Found: C,
40.15; H, 5.67; N, 2.59.
ꢀ, deg
V, Å3
Z
T, °C
F
calc, g/cm3
F(000)
radiation
µ, cm-1
transmn factors
2θmax, deg
total no. of reflns
no. of unique reflns
Rmerge
no. with I g σ(I)
no. of variables
R
NMR Reaction of Cp(NAr′)Ta(PMe3)2 with HSiMePhCl.
HSiMePhCl was added by syringe to an equivalent of
Cp(NAr′)Ta(PMe3)2 dissolved in C6D6. The spectrum recorded in
about 10 min showed clean formation of a mixture of two
diastereomers of Cp(NAr′)Ta(PMe3)(H)(SiMePhCl) (7c). IR (Nu-
jol): νTa-H ) 1652 cm-1. Upon heating for several hours at 65 °C,
these products transform into other compounds, including two
isomers of the rearranged product Cp(Ar′N)Ta(PMe3)(Cl)-
(SiMePhH) (8), characterized by their SiH quartets at 6.23 ppm
(J(H-H) ) 4.1 Hz) and 6.15 ppm (J(H-H) ) 4.1 Hz) coupled to
the methyl groups at 1.19 and 0.96 ppm, respectively. No signals
that might be assigned to an agostic species were observed.
First Isomer of 7c. 1H NMR (C6D6): δ 8.10 (d, J(H-H) ) 7.2
Hz, 4, both isomers, o-Ph), 7.34 (pt, J(H-H) ) 7.4 Hz, 2, p-Ph),
7.2-7.15 (m, both isomers, p-Ph), 7.02 (d, J(H-H) ) 7.2 Hz,
C6H3), 6.82 (pt, J(H-H) ) 7.5 Hz, C6H3), 5.65 (d, J(H-P) ) 63.1
Hz, 1, Ta-H), 5.30 (s, 5, Cp), 2.37 (s, 6, C6H3Me2), 1.40 (s, 3,
Me), 0.85 (d, J(H-P) ) 4.5 Hz, both isomers and free PMe3 are
in exchange). 13C{1H} NMR (C6D6): δ 156.4 (s, i-Ph), 150.4 (s,
i-Ar′), 134.1 (s, o-Ph), 127.8 (s, m-Ar′), 127.5 (p, m-Ph), 121.8
(p-Ar′),100.8 (Cp), 21.2 (s, Ar′), 20.2 (d, J(P-C) ) 2.9 Hz, PMe3),
15.0 (d, J(P-C) ) 2.0 Hz, SiMe). 31P{1H} NMR (C6D6.): δ -1.96
(s). 29Si gHMQC NMR (C6D6): δ 90.0.
0.0789
0.0798
1.1289
0.0087
Rw
GOF
max ∆/σ
(C6D6):
δ
106.1 (J(Si-H)
)
41 Hz). Anal. Calcd for
C17H27Cl2TaPNSi (556.3203): C, 36.70; H, 4.89, N 2.52. Found:
C, 36.60; H, 4.93; N, 2.50.
Cp(Ar′N)Ta(PMe3)(H)(SiPh2Cl) (7f). To a purple solution of
Cp(Ar′N)Ta(PMe3)2 (0.082 g, 0.155 mmol) in 10 mL of ether was
added 0.2 mL (1.02mmol) of HSiPh2Cl. The solution was allowed
to stand for 2 days at room temperature to produce well-shaped
yellow crystals. The crystals were filtered, washed by 2 mL of ether,
and dried. Yield: 0.039 g (0.059 mmol, 38%). IR (Nujol): νTa-H
)
1676 cm-1. 1H NMR (C6D6): δ 8.15 (d, J(H-H)) 8.1 Hz, 2, o-Ph),
8.11 (d, J(H-H) ) 8.1 Hz, 2, o-Ph), 7.26 (pt, J(H-H) ) 6.9 Hz,
4, m-Ph), 6.94 (d, J(H-H) ) 7.2 Hz, C6H3), 6.77 (pt, J(H-H) )
7.3 Hz, C6H3), 6.32 (d, J(H-P) ) 63.3 Hz, 1, Ta-H), 5.39 (d,
J(H-P) ) 1.5 Hz, 5, Cp), 2.07 (s, 6, C6H3Me2), 0.91 (d, J(H-P)
) 8.7 Hz, PMe3). 13C{1H} NMR (C6D6): δ 135.7, 135.3, 134.8,
134.7, 128.5, 127.7, 127.5, 121.9 (Ar and Ph), 100.9 (Cp), 20.9
(C6H3Me2), 20.2 (d, J(H-P) ) 30.9 Hz, PMe3). 31P{1H} NMR
(C6D6): δ -2.0 (s). 28Si NMR (C6D6): δ 85.5 (d, J(Si-P) ) 11.4
Hz). Anal. Calcd for C26H34ClNTaPSi (660.0466): C, 50.95; H,
5.19, N 2.12. Found: C, 49.68; H, 5.31; N, 2.15.
1
Second Isomer of 7c. H NMR (C6D6): δ 7.26 (pt, J(H-H) )
7.7 Hz, 2, m-Ph), 6.95 (d, J(H-H) ) 7.3 Hz, C6H3), 6.77 (pt,
J(H-H) ) 7.3 Hz, C6H3), 5.60 (d, J(H-P) ) 55.0 Hz, 1, Ta-H),
5.50 (s, 5, Cp), 2.07 (s, 6, C6H3Me2), 1.16 (s, 3, Me). 13C{1H}
NMR (C6D6): δ 156.2 (s, i-Ph), 150.4 (s, i-Ar′), 134.5 (s, o-Ph),
127.8 (s, m-Ar′), 127.4 (p, m-Ph), 100.4 (Cp), 20.9 (s, Ar′), 19.8
(d, J(P-C) ) 2.9 Hz, PMe3), 15.2 (d, J(P-C) ) 2.0 Hz, SiMe).
31P{1H} NMR (C6D6): δ -2.03 (s). 29Si gHMQC NMR (C6D6): δ
87.4 (s).
Experimental Determination of the Sign of J(Si-H). The
silicon-hydride coupling constants were measured from the
29Si-1H satellites in the 1H NMR spectra. The sign of the coupling
constant was determined from a spin-tickling experiment where
the 29Si satellites in the 1H spectrum of the proton terminally
attached to silicon were irradiated and the effect on the 29Si satellites
of the nonclassically bonded hydride was observed. This showed
that in Cp(ArN)Ta(PMe3)(Ha)(SiMePhHb) (2a) the signs of
1J(Si-Hb) (known to be negative, i.e., 1K(Si-Hb)27 has a positive
sign)28 and J(Si-H) are opposite. This observation establishes the
positive sign of the J(Si-Ha) in this compound: J(Si-Ha) ) +14
Hz (i.e., K(Si-Ha) has a negative sign). From a similar spin-tickling
experiment, by comparison to the negative sign of J(Si-Ha) in 2a,
Cp(Ar′N)Ta(PMe3)(H)(SiMeCl2) (7d). To
a solution of
Cp(NAr′)Ta(PMe3)2 (0.275 g, 0.53 mmol) in 30 mL of ether was
added 0.2 mL (1.9 mmol) of HSiMeCl2. The color immediately
changed to red. The solution was left at room temperature overnight
to give a yellow solution and a small amount of gray precipitate.
The solution was filtered and cooled to -80 °C to produce yellow
crystals (0.096 g, 0.172 mmol). The crystals were filtered and dried.
The second crop was obtained by keeping the concentrated mother
liquor at -80 °C (0.04 g, 0.072 mmol). Total yield: 46%. IR
(Nujol): νTa-H ) 1650 cm-1. 1H NMR (C6D6): δ 6.96 (d, J ) 7.2
Hz, 2 H, C6H3), 6.78 (pt, J ) 7.2 Hz, 1 H, C6H3), 6.00 (d, J(P-H)
) 64.8 Hz, 1 H, Ta-H), 5.59 (d, J(P-H) ) 1.5 Hz, 5 H, Cp),
2.27 (s, 6 H, C6H3Me2), 1.52 (s, 3, SiMe), 0.81 (d, J(P-H) ) 9.0
Hz, 9 H, PMe). 13C{1H} NMR (C6D6): δ 132.5, 124.1, 122.3 (all
Ar′), 101.5 (Cp), 21.0 (C6H3Me2), 19.5 (d, J(P-C) ) 31.5 Hz,
PMe), 18.7 (SiMe). 31P{1H} NMR (C6D6): δ -1.2 (s). 29Si NMR
2
the sign of J(Si-P) was found to be negative. In this case, the
proton NMR spectrum of the tantalum hydride was observed while
(27) Harris, R. K. Nuclear Magnetic Resonance Spectroscopy; Longman
Scientific and Technical: New York, 1983; p 221.
(28) Schrami, J.; Bellama, M., In Determination of Organic Structures
by Physical Methods; Nachod F. C., Zuckerman J. J., Randall E. W., Eds.;
Academic: New York, 1976; Vol. 6, pp 203-269.

Tantalum Silyl Hydrides
Organometallics, Vol. 27, No. 22, 2008 5977
low-power continuous irradiation was applied sequentially at the
positions of the low- and high-frequency 29Si satellites of the proton-
augmented by the p-polarization function (6-31G(d,p) basis set)
was used for the hydride H atom. The Hay-Wadt effective core
potentials (ECP) and the corresponding VDZ basis sets were used
for the Ta atoms.31
2
coupled 31P resonance. Given the negative sign of J(Si-P), the
negative sign of the J(Si-H) in 2d and 2e can be easily determined
by a similar spin-tickling experiment, which shows that the sign
X-ray Structure Analyses. The crystals of 7d were grown from
etherial solutions by cooling to -30 °C. The crystals were mounted
in a film of perfluoropolyether oil on a glass fiber and transferred
to a Siemens three-circle diffractometer with a CCD detector
(SMART system). The data were corrected for Lorentz and
polarization effects. The structure was solved by direct methods
and refined by full-matrix least-squares procedures (Table 4).32 All
non-hydrogen atoms were refined anisotropically; the hydrogen
atoms except hydride were placed in calculated positions and refined
in a ”riding” model. The hydride position has not been determined.
2
of J(Si-H) is the same as that of J(Si-P).
DFT Calculations. All calculations were carried out with the
Gaussian 03 program package29 using DFT applying Becke’s three-
parameter hybrid exchange functional in conjunction with gradient-
corrected nonlocal correlation functional of Perdew and Wang
(B3PW91).30 The compound basis set used for the calculation
consisted of the 6-31G(d) basis set for the Si, P, N, and Cl atoms,
6-31G for the carbon atoms and the silyl hydrogens, and the 3-21G
basis set for the H atoms of Cp ring and Me groups. The basis set
Acknowledgment. This work was supported through a
Royal Society (London) joint research grant to G.I.N. and
P.M., by EPSRC awards to S.R.D., and an NSERC grant to
G.I.N. S.K.I. and A.G.R. are grateful to RFBR for a research
grant.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.;
Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.;
Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski,
J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03; Gaussian, Inc.:
Wallingford, CT, 2004.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
OM800553Y
(30) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Perdew, J. P.;
Burke, K.; Wang, Y. Phys. ReV. B 1996, 54, 16533.
(31) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(32) SHELXTL-Plus, Release 5.10; Bruker AXS Inc.: Madison, WI,
1997.