The Electronic Structures of A2Y4 Molecules

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 s-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, C2O42-, B2F4, B2Cl4, C2F4, C2Cl4, S2O42-, 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 as, related to the coulomb and resonance parameters of Hückel molecular-orbital theory. General trends in the value of as 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.