A molecular orbital treatment of the basicity of oxyanion units

The basicity of the oxidic groups AlO₄^5?, SiO₄^4?, PO₄^3?, SO₄^2?, ClO₄^?, can be regarded in terms of electron donor power and expressed numerically as the optical basicity, λ. The λ value depends upon the oxidation number of the central atom and its basicity moderating parameter, γ. The results of (modified) self-consistent field CNDO calculations, presented and discussed in this paper, indicate the simple relationship that the optical basicity value is directly proportional to the negative charge borne by the oxygen atoms of the oxyanion. It is apparent that the basicity moderating parameter is a measure of the ability of the central atom to withdraw charge from the oxygen atoms. The results also indicate that the electron density at the “surface” of the oxyanion originates not only from the oxygens but also from the central atom, this effect increasing with decreasing oxidation number of the central atom.