Reversible neutral dissociation of the N-Si dative bond in hexacoordinate hydrido complexes of silicon

Hexacoordinate silicon dichelates with hydrido and methyl monodentate ligands were prepared by transsilylation. The temperature-dependent ²⁹Si NMR spectra of toluene-d₈ or CDC1₃ solutions provide evidence for a reversible, neutral dissociation of the dative N-Si bond, accompanied by a shift of the ²⁹Si resonance from high field (hexacoordinate) to lower field (pentacoordinate) with increasing temperature. When a phenyl group is attached directly to silicon (replacing methyl), the N-Si dissociation is prevented, presumably due to electron withdrawal by the phenyl. Two of the silicon complexes were characterized by crystallographic analysis. Quantum chemical calculations at the RB3LYP/6-31G(d) level confirm the stability of an N-Si dissociated species (1.03 kcal mol- ¹ lower in energy than the hexacoordinate dichelate) and rale out the alternative possibility of an O-Si zwitterionic dissociation, thus supporting the mechanism suggested by the experimental results.