The molecular orbital description of SN2 reactions at silicon centres

Bimolecular nucleophilic substitution at a silicon centre has been investigated by the use of ab initio molecular orbital theory in an examination of the attack of hydride and fluoride anions upon silane and silyl fluoride, respectively. In particular, the role of polarization functions in determining the magnitude of reaction barriers was explored. The inclusion of diffuse functions in the calculations was necessary in order to correctly describe the anionic species. Comparative calculations were performed for the corresponding carbon systems. It is concluded that the carbon systems show barriers to reaction since their reaction complexes are of higher energy than the reactants, unlike the silicon systems where the reaction complexes are of lower energy. The carbon reaction complexes exhibit elongated bonds to the entering and leaving groups, which is expected from the traditional picture of bimolecular nucleophilic substitution, in contrast to the silicon complexes where only small changes from the reactant bond lengths occur.