Molecular Orbital Approach to Cyclic Anionic Pentaoxysilicates, Fluorosilicates, and Isoelectronic Phosphoranes. Structural Comparisons¹,²

Geometries and relative energies from ab initio calculations using Gaussian 92 on five-coordinated anionic cyclic pentaoxysilicates and isoelectronic cyclic pentaoxyphosphoranes containing saturated five- and six-membered rings in trigonal bipyramids (TBP) are compared with structural results from X-ray diffraction and VT NMR studies. The theoretical study is extended to cyclic C 4 H 4 SiF 3 -, C 4 H 8 SiF 3 -, and isoelectronic phosphorus species in trigonal-bipyramidal geometries. In agreement with structural results, the saturated five- and six-membered rings in pentaoxy species are located on the potential energy surface as local energy minima with the rings occupying axial−equatorial (a−e) sites. In the cyclic anionic organofluorosilicates and organofluorophosphoranes, the five-membered saturated rings reside in diequatorial (e−e) sites, whereas the five-membered unsaturated rings are located in (a−e) sites. It is suggested that these cyclic derivatives provide useful models for nucleophilic displacement reactions. Low calculated axial bond overlap populations found for anionic silicon compared to isoelectronic phosphorus entities correlate with accompanying longer axial Si−O and Si−F bond lengths. It is concluded that the long axial bond lengths accompany weaker bond strengths, as indicated by the ab initio results and represent a major factor responsible for enhanced reactivity of five-coordinated anionic organosilicon compounds. This bond length factor, associated with the reduced nuclear charge on silicon, suggests greater fluxional character, which is consistent with the lower ligand exchange activation energies involving ring reorientations found for five-coordinated silicon compared to that for phosphorus. Bond-lengthening effects are found in the series RR‘SiF 3 - that correlate with the bulkiness of the organo substituents. Structural nonrigidity leads to axial bond shortening as the structure traverses from trigonal bipyramidal to square pyramidal, a feature which may reduce reactivity in nucleophilic displacement reactions for pentacoordinated molecules thus displaced.