A semiempirical correlation between the optical band gap of hydroxides andthe electronegativity of their constituents

In analogy with previous results on anhydrous oxides, a correlation is prop osed between the optical band gap of hydroxides and the electronegativities of their constituents. Based on the experimental results on passive hydrat ed layers on metals obtained in our laboratory and the literature data, it is found that the hydroxide band gap varies with the square of the differen ce between the electronegativities of the metallic cation and the hydroxyl group. Like in the case of anhydrous oxides, two different interpolation li nes have been found for sp-metal and d-metal hydroxides, respectively. The proposed correlations predict semiconducting or insulating behavior even fo r the most electronegative metals, sat variance with the case of oxides of noble metals, for which the observed metallic behavior is foreseen.