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

In analogy with previous results on anhydrous oxides, a correlation is proposed between the optical band gap of hydroxides and the electronegativities of their constituents. Based on the experimental results on passive hydrated 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 difference between the electronegativities of the metallic cation and the hydroxyl group. Like in the case of anhydrous oxides, two different interpolation lines have been found for sp-metal and d-metal hydroxides, respectively. The proposed correlations predict semiconducting or insulating behavior even for the most electronegative metals, sat variance with the case of oxides of noble metals, for which the observed metallic behavior is foreseen.