Abstract
The character of singlet (C3N2H5)CuO2 ranges smoothly between copper(III) peroxide and copper(II) superoxide with variation of the electronic character of the supporting β-diketiminate ligand. Over the range of the variation, multireference second-order perturbation theory predicts the 1A1 singlet state always to be lower in energy than the lowest triplet state (3B1). The multideterminantal character of the biradical-like superoxide mesomer causes density functional theory sometimes to fail badly in predicting the relative energies of these same states, although its predictions of other properties, such as geometry, are of good quality.