Ligand-field strength and symmetry-restricted covalency in Cu^II complexes - A near-edge X-ray absorption fine structure spectroscopy and time-dependent DFT study

The low-lying empty orbitals of bis(acetylacetonato)copper have been probed by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Cu L2,3 edges and time-dependent density functional theory (TDDFT) calculations within the Tamm-Dancoff approximation (TDA) coupled to the relativistic zeroth-order regular approximation including spin-orbit effects (SO-ZORA TDDFT-TDA). Both the relative positions of NEXAFS features and their linear dichroism are satisfactorily reproduced. Moreover, a comparison with literature data pertaining to two other square-planar Cu^II complexes ([CuCl4]^2- and copper phthalocyanine) provided information about how metal-to-ligand charge-transfer transitions associated with excitations from Cu^II 2p orbitals to low-lying, ligand-based ?* molecular orbitals in a simplified one-electron picture may contribute to the Cu^II L2,3 edge intensity and, thus, weaken its believed relationship with the Cu^II-ligand symmetry-restricted covalency.