Rhodium silyl hydrides in oxidation state+5: Classical or nonclassical?

Four families of silyl hydride complexes of rhodium in the formal oxidation state +5 were investigated by means of DFT calculations, supplemented by the calculation of Mayer bond indices and Si-H coupling constants. In each case some degree of interligand Si-H interaction has been found. In the compounds CpRh(SiMe3)(2)H-2 (1), CpRh(SiMe3)(3)H (2), CpRh(SiMe3)(2)(SiEt3)H (3), and [Cp(Me3P)Rh(SiMe3)(2)H](+) (4) the hydride(s) interact(s) with both silyl ligands. Relaxed potential energy scans indicate that the potential energy surface is extremely flat. It takes only 1 kcal center dot mol(-1) to compress the Si-H bond from 2.3 angstrom to 2.0 angstrom in 1, and 2 kcal center dot mol(-1) to compress the Si-H distance from 1.990 angstrom to 1.70 angstrom in complex 2. ONIOM calculations of the compound CpRh(SiMe3)(3)H and optimization of the lowest energy conformers of complex CpRh(SiMe3)(2)(SiEt3)H show that their geometries are largely determined by steric effects. Increasing steric hindrance promotes Si-H interactions because they result in longer Rh-Si bond lengths, leading to the relief of steric strain. The same conclusion has been drawn from the comparison of complexes [Cp(PMe3)Rh(SiMe3)(2)H](+) and [Cp*(PMe3)Rh(SiMe3)(2)H](+), the latter compound having stronger Si-H interactions. A very low barrier of 1.9 kcal center dot mol(-1) (on the Delta G degrees(298) scale) has been found for the hydride shift in complex 2, accounting for its fluxionality. In contrast, the complexes [Cp(PMe3)Rh(SiMe3)(2)H](+) (4) and [Cp(PMe3)Rh(SiMe3)H-2](+) (8) are not fluxional because the hydride migration is accompanied by a highly unfavorable loss of one of the Si center dot center dot center dot. H interactions. The calculated Si-H coupling constants are negative when the silyl and hydride ligands are cis (consistent with the presence of Si-H bonding) and positive for the trans pairs (no Si-H bonding). The magnitude of the calculated J(H-Si) is in very good accord with experimental data, when the latter are available.