Overlap polarizability of a chemical bond: a scale of covalency and application to lanthanide compounds

We introduce the concept of overlap polarizability of a single directional chemical bond and propose an expression for this quantity. The concept of ionic specific valence is also introduced, as a consequence of a relation, involving the bond force constant, between the overlap polarizability and the square of the overlap charge. It is proposed that the overlap polarizability can be used to define an ordinal scale of covalency. When applied to lanthanide compounds, each pair lanthanide-ligating atom being regarded as a diatomic-like molecule, these concepts lead to a remarkable result in which an expression for the charge factors, appearing in the simple overlap model for the ligand field, can be obtained when these factors are identified with the ionic specific valences of the ligating atoms. An ordinal scale of covalency for a few representative lanthanide compounds correlates well with the nephelauxetic effect. (C) 2002 Elsevier Science B.V. All rights reserved.