Pseudo-halide complexes of transition metals. Part II. Spectra, structure, and nature of bonding

Abstract

The available spectral data of complexes of transition-metal ions with halide and pseudo-halide ligands are correlated with their structures. Ligand optical electronegativities are compared with the nephelauxetic effect of a complex so that single and multiple ligand–metal bonding can be differentiated. The ligand electronegativities display a strong correlation both with co-ordination number and, for any one such number, the different possible symmetries. High ligand electronegativities, χ_L, are associated with both high co-ordination numbers and high complex symmetries. Ligands with lower values of χ_L promote lower co-ordination numbers and distorted symmetries. This is rationalized in terms of the charge balance requirements of the metal ion and the covalence of the ligand–metal bonds. Multiple bonding in complexes appears to stabilize the more ionic stereochemistries. The two effects of bond covalence and order are used to account for the observed symmetries of anionic complexes of divalent metal ions and, in particular, the various structures of four-co-ordinate nickel(II) and copper(II) compounds.