A simple electrostatic model for trisilylamine: Theoretical examinations of the n ->sigma* negative hyperconjugation, p pi -> d pi bonding, and stereoelectronic interaction

A block-localized wave function method was used to examine the stereoelectr onic effects on the origin of the structural difference between trisilylami ne and trimethylamine. The pyramidal geometry of trimethylamine along with its high basicity is consistent with thr traditional VSEPR (valence shell e lectron-pair repulsion) model for sigma bonding. On the other hand, in tris ilylamine, the silicon d orbitals make modest contribution to the electroni c delocalization, although the key factor in charge delocalization is still n(N)-->sigma(SiH)* negative hyperconjugation. Interestingly, the gain in p (pi)-->d(pi) bonding stabilization is offset by a weaker negative hyperconj ugation effect in trisilylamine, resulting in an overall smaller delocaliza tion energy (-18.5 kcal/mol) than that in trimethylamine (-23.9 kcal/mol), which contains little p(pi)-->d(pi), bonding character. Significantly, beca use of the relatively low electronegativity of silicon, the N-Si bond is mu ch more polar than the N-C bond. Weinhold's natural population analyses of the BLW and HF wave functions for these compounds reveal that the origin of the planar geometry of trisilylamine is due to the polar sigma-effect that yields significant long-range electrostatic repulsion between the silyl gr oups. In addition, it was found that only the most electronegative substitu ents such as F and OH can result in a pyramidal geometry at the nitrogen ce nter for silylamines. This is in good accord with the recent X-ray structur e of a pyramidal silylamine, N(CH3)(OCH3)(SiH3).