FLUORINE AND CHLORINE APICOPHILICITIES IN FIVE-COORDINATED PHOSPHORUS AND SILICON COMPOUNDS VIA MOLECULAR ORBITAL CALCULATIONS. A MODEL FOR NUCLEOPHILIC SUBSTITUTION

Abstract

Abstract Comparative ab initio molecular orbital calculations on pentacoordinated phosphorus and silicon model states are used to establish apicophilicities in a series of trigonal-bipyramidal molecules composed of all combinations of the ligand atoms, hydrogen, chlorine, and fluorine. In agreement with experimental observations, it is shown that chlorine is in general more apicophilic than fluorine except when the collective electronegativity of equatorial atoms is increased sufficiently, e.g., as in the PCl n Fs-n series. The crossover in halogen apicophilicity for silicon has an even greater electronegativity requirement in equatorial sites than that for phosphorus. Further, as applied in nucleophilic substitution reactions, the apicophilicity of the leaving group is diminished by the composition of equatorial ligands in the order Cl > F > H. In contrast, the apicophilicity of the leaving group is enhanced by the opposite apical atom in this same order, Cl > F > H, in all cases except that in which the...