| Abstract
| - A block-localized wave function method was used to examine the stereoelectronic effects on theorigin of the structural difference between trisilylamine and trimethylamine. The pyramidal geometry oftrimethylamine along with its high basicity is consistent with the traditional VSEPR (valence shell electron-pair repulsion) model for σ bonding. On the other hand, in trisilylamine, the silicon d orbitals make modestcontribution to the electronic delocalization, although the key factor in charge delocalization is still nN→σSiH*negative hyperconjugation. Interestingly, the gain in pπ→dπ bonding stabilization is offset by a weaker negativehyperconjugation effect in trisilylamine, resulting in an overall smaller delocalization energy (−18.5 kcal/mol) than that in trimethylamine (−23.9 kcal/mol), which contains little pπ→dπ bonding character. Significantly,because of the relatively low electronegativity of silicon, the N−Si bond is much more polar than the N−Cbond. Weinhold's natural population analyses of the BLW and HF wave functions for these compounds revealthat the origin of the planar geometry of trisilylamine is due to the polar σ-effect that yields significant long-range electrostatic repulsion between the silyl groups. In addition, it was found that only the most electronegativesubstituents such as F and OH can result in a pyramidal geometry at the nitrogen center for silylamines. Thisis in good accord with the recent X-ray structure of a pyramidal silylamine, N(CH3)(OCH3)(SiH3).
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