Improved scales for metal ion softness and toxicity

Ten scales relating to chemical hardness or softness of metal ions were compiled. These included eight published scales such as those of Pearson, Ahrland, Klopman, and Misono. Another scale consisted of the logs of the solubility products of metal sulfides, and yet another was a consensus scale constructed from -log K values for metal ion binding to seven soft ligands. These 10 scales were normalized and averaged. The resulting consensus scale for softness (sigma(Con)) appeared to be superior to any of the 10 scales used in its construction based on correlations among the scales. Other possible indicators of softness were examined, including the standard electrode potential (E(0)) and the bulk metal density (rho(Metal)), both of which were also superior to most of the 10 scales just mentioned. Vales for sigma(Con) may be computed from E(0), rho(Metal), and the first ionization potential (I(P)), R(2) = 0.867, for the equation sigma(Con) = aE(0)I(P) + brho(Metal). A consensus scale for toxicity (T(Con)) derived from studies with many different taxa correlated well (R(2) = 0.807) with sigma(Con) computed from the preceding equation, but incorporation of ion charge (Z) into the following equation, T(Con) = asigma(Con) + bsigma(Con)Z + cZ, increased R(2) to 0.923. Substitution of other softness scales for sigma(Con) into equations to predict T(Con) reduced the value of R(2). Thus, sigma(Con) appears to be a superior scale for metal ion softness and toxicity, the latter being an interactive function of both softness and charge.