The Myth of d(8) 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 d(8) 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.