Sulfur K-Edge X-ray Absorption Spectroscopy as a Probe of Ligand−Metal Bond Covalency:  Metal vs Ligand Oxidation in Copper and Nickel Dithiolene Complexes

A combination of Cu L-edge and S K-edge X-ray absorption data and density functional theory (DFT) calculations has been correlated with 33S electron paramagnetic resonance superhyperfine results to obtain the dipole integral (I s) for the S 1s→3p transition for the dithiolene ligand maleonitriledithiolate (MNT) in (TBA)2[Cu(MNT)2] (TBA= tetra-n-butylammonium). The results have been combined with the I s of sulfide derived from XPS studies to experimentally obtain a relation between the S 1s→4p transition energy (which reflects the charge on the S atom, ) and the dipole integral over a large range of . The results show that, for high charges on S, I s can vary from the previously reported I s values, calculated using data over a limited range of . A combination of S K-edge and Cu K- and L-edge X-ray absorption data and DFT calculations has been used to investigate the one-electron oxidation of [Cu(MNT)2]2- and [Ni(MNT)2]2-. The conversion of [Cu(MNT)2]2- to [Cu(MNT)2]- results in a large change in the charge on the Cu atom in the molecule ( ) and is consistent with a metal-based oxidation. This is accompanied by extensive charge donation from the ligands to compensate the high charge on the Cu in [Cu(MNT)2]- based on the increased S K-edge and decreased Cu L-edge intensity, respectively. In contrast, the oxidation of [Ni(MNT)2]2- to [Ni(MNT)2]- results in a small change in , indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, (singly occupied molecular orbital), with predominant ligand character.