Qualitative valence bond theory: Speculations concerning the origin of some bond-length variations

Abstract

Qualitative valence bond rationalizations for the origin of NN,SS and CC bond-length variations are redescribed for N 2O 4, S 2O 4 2− and C 2O 4 2−, and extended to S 2O 5 2−, S 2O 6 2−, C 6H 5SO 2− SO 2C 6H 5 and S 2O 2. Attention is focus on the effect of oxygen lone-pair electron delocalization, and on the resulting long-bond structures and increased-valence structures that are obtained. The NN bond length of 2.237 Å for C 2v symmetry N 2O 2 is contrasted with the CC bond length of 1.525 Å for isoelectronic (HCO) 2. If both bonds were normal single bonds, differences in orbital hybridization would lead to an appreciable weakening of the NN bond relative to the CC bond. A further weakening (and concomitant lengthening) of these bonds occurs when oxygen lone-pair electrons delocalize into the orbitals that form these bonds. It is suggested that the latter type of bond weakening for N 2O 2 can be more than compensated for by the development at large NN internuelear separations of strong NO Pauling ‘3-electron bonds’, which occur in the increased-valence structure. ▪ In contrast, because the CC single bond of CHOCHO is relatively strong, only a slight weakening and lengthening of it through oxygen lone-pair electron delocalization is energetically favourable. A Pople-type SCF molecular orbital procedure, with configuration interaction, is used to help utilize aspects of the photoelectron spectrum for N 2O 2, in order to provide an empirical estimate of the weight for the increased-valence structure.