INCREASED-VALENCE STRUCTURES AND HYPERVALENT MOLECULES

Hypervalent molecules may involve the use of increased-valence structu res to provide valence bond descriptions of their electronic structure . For electron-rich molecules with four electrons distributed among th ree overlapping (nuclear-centered) atomic orbitals, the increased-vale nce structures are Y-A . B and Y . A-B. Each structure involves a frac tional electron-pair bond and a one-electron bond. It is deduced that the Armstrong-Perkins-Stewart valence of the A atom is able to exceed unity in each of these structures when the three bonding electrons occ upy nonorthogonal localized molecular orbitals. It is also shown that increased valence for the A atom does not occur when the four electron s occupy localized molecular orbitals to give the valence-bond structu re Y-A-B with three overlapping atomic orbitals, and the same number o f orbital variational parameters as occurs in the wave functions for e ither of the increased-valence structures. The results of ab initio va lence bond calculations with minimal basis sets are reported for H(3)( -)l, CH5-, HF2-, F-3(-), CIF3, and FF3, and the resulting wave functio ns for resonance between six canonical Lewis structures are related to those for resonance between the two increased-valence structures. The use of the latter structures to indicate how electronic reorganizatio n proceeds via one-electron delocalizations for S-N2 reactions is rede scribed, and an elementary argument is presented to deduce that this c lass of reactions cannot involve the delocalization of a pair of elect rons in concert from the nucleophile. Increased-valence wave functions are used to deduce an expression for the avoided crossing for the tra nsition state of the identity S-N2 reaction. (C) 1996 John Wiley & Son s, Inc.