The Myth of d⁸ Copper(III)

Seventeen Cu complexes with formal oxidation states ranging from Cu^I to Cu^III are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally Cu^III species is extremely covalent, resulting in Cu K-edge and L_2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L_2,3-edge data reveals that all formally Cu^III species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d⁸ description for these formally Cu^III species is inaccurate. This study provides an alternative explanation for the competence of formally Cu^III species in transformations that are traditionally described as metal-centered, 2-electron Cu^I/Cu^III redox processes.