Stereo-Electronic Effects in Substituted Phosphines: a Molecular Orbital Study

Abstract

Substituent effects from directly bonded and remote groups in phosphines have been studied by ab initio molecular orbital methods in an attempt to discriminate between the electronic and steric effects of attached groups and determine how they are transmitted to phosphorus. Results of calculations are compared with experimentally determined basicities and acidities, ionization energies, inversion barriers and ³¹P N.M.R. results. The electronic consequences of steric effects are comparable in importance to direct electronic effects; in the PH_n(CH₃)_3-n, series steric effects account for about half of the substituent effect on basicities and ionization potentials. The effects of steric bulk, modelled by bond angle constraint, are concentrated in the HOMO; by contrast, electronic effects on the HOMO are often minimal. Conformational changes include changes in geometry but the electronic response exceeds what would be expected for geometrical change alone and in NH_2- and OH-substituted model compounds of the PH₂X and PX₃ series may be as great as that produced by changing the substituent. In remote substitution the effects are mainly due to charge displacement at phosphorus. Directly bound groups more often exert their effects by altering the balance between s and p orbitals in the bonding and in the non-bonding parts of the electron distribution but in ways which preclude correlation with electronegativity or other single-parameter measures of substituent properties. Donor/acceptor interactions are weaker in phosphorus(III) compounds than in the first-row analogues but the response to substitution in the density profile is much stronger around the more polarizable second-row element which explains why the alkyl substituent effect is greater for the donor properties of phosphines than amines while the effects of fluoro-substitution are weaker.