Stability of the oxidation states of copper

An electronic model is developed to understand the empirical observation that high oxidation states of transition metals may be stabilized as their oxide by the addition of an electropositive element. Thus, although copper is known as Cu(I) and Cu(II) in compounds with oxygen, no binary Cu(III) oxide (Cu2O3) is presently known. Cu(III), however, may be stabilized as a ternary oxide as in KCuO2. Interpretation of the results of LMTO calculations on a series of copper oxides leads to a model which focuses on the interaction between copper and oxygen orbitals by considering the energy gap between them. The orbitals on silver and gold interact more strongly with the oxygen levels than the contracted d orbitals of the first-row element copper. The interactions between gold and oxygen are enhanced by relativistic effects. Thus the strength of the interaction increases in the order Cu < Ag < Au. Of the series, Cu2O3 has not yet been synthesized, Ag2O3 has only recently been made with great difficulty, but Au2O3 is well-known as a stable system. In (KCuO2)-O-III the calculations show that there is effective charge transfer from potassium to oxygen, which leads to a raising of the oxygen levels, a decrease in the metal spd-oxygen 2p separation, and the generation of a stronger interaction between copper and oxygen, i.e., an increase in Cu-O covalency. Calculations on copper(II) halides show that there is nothing inherently peculiar concerning electronic structure of the unknown solid CuI2, but it is probably just unstable with respect to (CuI)-I-I and elemental iodine.