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, 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 aσ,
related to the coulomb and resonance parameters of H�ckel molecular-orbital
theory. General trends in the value of aσ
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.