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(SiMe 3 ) 2 H 2 ( 1 ), CpRh(SiMe 3 ) 3 H ( 2 ), CpRh(SiMe 3 ) 2 (SiEt 3 )H ( 3 ), and [Cp(Me 3 P)Rh(SiMe 3 ) 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·mol - 1 to compress the Si−H bond from 2.3 Å to 2.0 Å in 1, and 2 kcal·mol - 1 to compress the Si−H distance from 1.990 Å to 1.70 Å in complex 2 . ONIOM calculations of the compound CpRh(SiMe 3 ) 3 H and optimization of the lowest energy conformers of complex CpRh(SiMe 3 ) 2 (SiEt 3 )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(PMe 3 )Rh(SiMe 3 ) 2 H] + and [Cp*(PMe 3 )Rh(SiMe 3 ) 2 H] +, the latter compound having stronger Si−H interactions. A very low barrier of 1.9 kcal·mol - 1 (on the scale) has been found for the hydride shift in complex 2, accounting for its fluxionality. In contrast, the complexes [Cp(PMe 3 )Rh(SiMe 3 ) 2 H] + ( 4 ) and [Cp(PMe 3 )Rh(SiMe 3 )H 2 ] + ( 8 ) are not fluxional because the hydride migration is accompanied by a highly unfavorable loss of one of the Si···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.