The Electronic Structures of A₂Y₄ Molecules

Abstract

A study of the electronic structures of A₂Y₄ molecules containing 34, 36, and 38 valence electrons has been made. An approximate VESCF, molecular- orbital method was used, attention being concentrated mainly on the delocalization of σ-electrons which are classically regarded as lone-pairs on the Y atoms. The results provide explanations of the main features of many of the observed AA- and AY-bond lengths and YAY-bond angles of N₂O₄, C₂O₄^2-, B₂F₄, B₂Cl₄, C₂F₄, C₂Cl₄, S₂O₄^2-, and N₂F₄. Other A₂Y₄ systems which have either not been fully characterized or not yet reported are also considered. The extent of lone-pair delocalization is shown to be governed by a parameter a^σ, related to the coulomb and resonance parameters of Hückel molecular-orbital theory. General trends in the value of a^σ can be predicted from values of the core charges of A and Y towards the o-electrons concerned. A more detailed "adjacent charge" rule emerges. It differs from the classical rule in that for A₂Y₄ systems, adjacent negative formal charges on the A atoms should not very greatly affect the AA-bond properties. Difficulties were encountered in consistently interpreting the properties of some A₂F₄ and A₂Cl₄ compounds. These deserve further attention.