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 S-33 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, Q mol S) and the dipole integral over a large range of Q mol S. 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 Q mol S. 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 (Q mol Cu) 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 Q mol Ni, indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, psi*(SOMO) (singly occupied molecular orbital), with predominant ligand character.