The Electron Localization Function (ELF) Description of the PO Bond in Phosphine Oxide

Abstract

The electron localization function1-5 (ELF) has been used to study the disposition of electron pairs in H3PO. Three attractors corresponding to lone pairs on the oxygen are found behind the oxygen (away from phosphorus) staggered with respect to the PH bonds. The nature of the isosurfaces for the lone pairs and the PO bond attractor is indicative of an ionic type of environment around oxygen and supportive of the oxygen lone pair polarization and back-bonding stabilization of the PO bond. The nature of the bonding in the PO bond of phosphine oxides (R3PO) has been of great interest for many years. It has been reviewed extensively by Gilheany6,7 who points out that both experiment and ab initio calculations generally agree that the PO bond is strong, polar, and as short as conventional PO double bonds. The role of d functions as polarization functions rather than primary valence orbitals is well established.8,9 Where differences arise is in the interpretation of the bond based on different approaches. Reed and Schleyer10 view the bonding as a donor-acceptor interaction with superimposed oxygen π orbital back-bonding with the degenerate H3P moiety antibonding orbitals (negative hyperconjugation11). Gordon and co-workers12-14 use energy-localized orbitals15-17 to picture the bond as one strong PO σ bond and three equivalent oxygen orbitals characterized primarily as lone pairs polarized toward phosphorus and staggered with respect to the PR bonds. At the same time, they12 and others18,19 derive a picture based on the Boys localization scheme20,21 that involves a single lone pair orbital on oxygen pointing away from the H3P group in H3PO and three bent or banana bonds strongly polarized toward oxygen. Reed and Schleyer10 employed natural localized molecular orbitals22,23 derived from a natural bond order analysis24 to conclude that the bonding was dominated by ionic interactions and negative hyperconjugation. The fact that three quite different orbital schemes arise from basically the same Hartree-Fock (HF) density illustrates the arbitrary nature of this subdivision of charge.