Revisions to expressions for atomic valencies for 3-electron 3-centre and 4-electron 3-centre valence bond structures

Abstract

With Coulson–Fischer type orbitals to accommodate the electrons of an A–B electron-pair bond, it is demonstrated that the A-atom and B-atom valencies need to be formulated as V ab= V ba=2 P aa−( P aa ) 2≡ P aa P bb rather than via the Wiberg closed-shell formula ( P ab) 2. Revised expressions are deduced for the A-atom valencies for the 3-electron 3-centre and 4-electron 3-centre VB structures, Y·A–B, Y ̇ · A– B and Y–A–B when the bonding electrons occupy the molecular orbitals ψ ya= y+ la and ψ ab= a+ kb. With atomic orbital overlap integrals omitted, it is calculated that the A-atom valence in each of these VB structures can exceed unity.