Rhodium Silyl Hydrides in Oxidation State +5: Classical or Nonclassical?

Abstract

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)2H2 (1), CpRh(SiMe3)3H (2), CpRh(SiMe3)2(SiEt3)H (3), and [Cp(Me3P)Rh(SiMe3)2H]+ (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 A to 2.0 A in 1, and 2 kcal·mol-1 to compress the Si−H distance from 1.990 A to 1.70 A in complex 2. ONIOM calculations of the compound CpRh(SiMe3)3H 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, leadin...