Si-H bond coordinated

The importance of a Si-H-M interaction in reaction intermediates was demonstrated in the studies of silane alcoholysis by an Ir complex (Eq. 2.29) [88]. Kinetic and mechanistic studies of silane alcoholysis catalyzed by [IrH2S2(PPh3)2]SbF6 (S = solvent) suggest that an unstretched silane 27 is an active intermediate. In this system the Ir(III) center carries a positive charge making the metal electrophilic. A Si-H bond coordinated to the electrophilic metal center would be activated without oxidative addition. The result is enhanced sensitivity to nucleophilic attack by... 

We are interested in the H-H bond length, so we specify the coordinate bonding those two atoms to the AddRedundant option so that its value will be included in the printout of the optimized structure (the Si-H bond lengths will be included by default). 

It was proposed (Johnson et al., 1987a) that this local lattice dilation is stabilized by the direct incorporation of hydrogen atoms through the coordinated formation of Si—H bonds. Results from SIMS (Section III. 1) and Raman spectroscopy (following) are consistent with this view. For example, the 60-min deuterium profile in Fig. 7(b) yields an integrated areal density of D in the near-surface peak of —1.7 x 1014 cm-2. The same deuteration conditions applied to this material produced 5 x 10n platelets per cm2 with an average diameter of 7 nm (Ponce et al., 1987). 

For this reaction, the reaction coordinate is simply the Si-H bond distance. Since the reverse of this reaction is radical-radical recombination and should have no activation energy barrier, the E for forward reaction would be expected to be AH,. 

The reaction coordinate again is simple it is the H-Si-H bond angle. In this case, however, the reverse reaction is a radical-molecule reaction, and we cannot make the a priori assumption that its activation energy would be zero. In fact, the literature is full of examples of radical-molecule reactions with large activation energies (Benson, 1976). As a result, we cannot also make the assumption that for the forward reaction E = AH as we did in the case of the Si-H bond fission reaction. At this point, we must resort to either quantum chemical calculations or experiments to resolve this issue. 

The silanol complex 57 exhibits a Si H M agostic interaction characterized by a /(Si-H) of 41 Hz and a Si-H distance of 1.70(7) It would be incautious to interpret such a low value of the Si-H coupling in terms of a significant Si-H bond activation, because the Si-H bond forms rather acute angles with the Si-C and Si-Si bonds (about 82 and 101°, respectively) and thus must have a considerable p character on silicon, which should contribute to the decrease of /(Si-H). The silanol ligand is -coordinate to ruthenium and the Ru-Si bond of 2.441(3) A is not exceptional, but the Si(SiMe3)3 deviates from the silanol plane by 19.0°, probably as a result of the Si-H interaction. Deprotonation of 57 by strong bases affords a neutral ruthenocene-like product. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

While the last elass of eomplexes considered in this section, the compounds 145, closely resemble the usual silane a-complexes, other multicenter H Si interactions discussed above have spectroscopic and structural features common to both the IHI and a-complexes. This enigmatic situation can be explained well by the structure 132 in terms of a a-coordination of the Si-H bonds of the hypervalent ligand (H +iSiX3)" 1) to metal, which thus includes both the hypervalent interaction of the silicon with the hydride atoms and the a-complexation of the Si-H bonds to metals. The key features of complexes with multicenter H Si interactions are summarized in Table VIII, where a comparison with the IHI and the residual H-Si interactions in silane a-complexes is given. 

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