NEW BOND-VALENCE SUM MODEL

A new expression is devised empirically to accommodate zero and some n egative oxidation states in the bond-valence sum approach. The method is worked out in detail for a number of homoleptic copper and nickel c omplexes of various coordinating atoms in several oxidation states of the metals. An implication of the expression is a linear variation bet ween 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 lengt hs. This is verified in Cu2+ X-6 chromophores for X = F, O, N and S. T he usefulness of the new expression in assessing the compatibility of a coordination sphere with an oxidation state of a metal ion is demons trated by exemplary applications to some inorganic complexes, azurin a nd urease.