A new expression is devised empirically to accommodate zero and some negative oxidation states in the bondvalence 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/req and 1/rax in octahedral MX 6 moieties, where req and tax 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. references therein; Donnay & Donnay, 1973; Brown, 1978, and references therein; O'Keeffe, 1989, and references therein; Brown, 1992, and references therein;.It has been realized from the beginning that v o. bears an inverse relation with the distance (r) between two atoms i and j -a shorter bond implies higher v,j, while a longer bond indicates lower vo.. However, the exact relation between vv and r o.is not yet known. Several empirical forms have been suggested over the years (Donnay &

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