Ligand-field stabilization energies of the first-row transition-metal dihalides and their bearing on the Irving–Williams rule

Abstract

A new analysis of the ligand-field stabilization energy which allows for contributions from the nephelauxetic effect, is shown to give satisfactory results for solid dihalides of the first transition series. In agreement with the nephelauxetic series, this contribution increases with the size of the halide ligand, and sometimes exceeds 50% in the diiodides. The new treatment seems to be an improvement upon those which equate ligand-field and d-orbital stabilization energies; it suggests that the value of Δ_fH^⊖(VI₂, s) recommended by the National Bureau of Standards is 30–40 kJ mol^–1 too negative. The formation of a solid dihalide from aqueous solution is used as a model for formation of an aqueous octahedral halogeno complex. It seems that the weak nephelauxetic effect of water as a ligand contributes to the emergence of the Irving–Williams order of stability in complexing and pseudo-complexing reactions for the oxidation states Mn^II→Zn^II in aqueous solution.