ENHANCED REACTIVITY OF PENTACOORDINATED SILICON SPECIES. AN AB INITIO APPROACH

Abstract

Molecular orbital calculations using GAUSSIAN 86 on tetracoordinated silicon species SiH _n F_4-n are compared with calculations on the anionic species formed by the addition of fluoride ion [SiH _n F_5-n ]. Similar calculations on isoelectronic tetra- and pentacoordinated phosphorus species [PHnF_4-n ]⁺ and PH _n F_5-n are also carried out. All bond lengths in the pentacoordinated species are longer than in the related tetracoordinated species, with the greater increase in bond lengths occurring in the axial positions from both the silicon and phosphorus series. Fluoride ion addition results in only slight changes in the positive charge on silicon and phosphorus and in an increase in negative charge on all substituents. These calculations are used to elucidate the enhanced reactivity of pentacoordinated silicon observed in nucleophilic reactions.