New Bond-Valence Sum Model

A new expression is devised empirically to accommodate zero and some negative oxidation states in the bond-valence sum approach. The method is worked out in detail for a number of homoleptic copper and nickel complexes of various coordinating atoms in several oxidation states of the metals. An implication of the expression is a linear variation between 1/r_eq and 1/r_ax in octahedral MX₆ moieties, where r_eq and r_ax are, respectively, the average equatorial and axial bond lengths. This is verified in Cu²⁺X₆ chromophores for X = F, O, N and S. The usefulness of the new expression in assessing the compatibility of a coordination sphere with an oxidation state of a metal ion is demonstrated by exemplary applications to some inorganic complexes, azurin and urease.