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

Abstract

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 (Is) 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 Is 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, QSmol) and the dipole integral over a large range of QSmol. The results show that, for high charges on S, Is can vary from the previously reported Is values, calculated using data over a limited range of QSmol. 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 (QCumol) 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 QNimol, indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, psiSOMO (singly occupied molecular orbital), with predominant ligand character.

Sulfanyl stabilization of copper-bonded phenoxyls in model complexes and galactose oxidase

“…7 B); the preedge energies are similar to sulfur-centered radicals reported by Martin-Diaconescu and Kennepohl (40). In metal-bonded sulfur complexes, the preedge features intensity reflects S p mixing due to covalency of the metal-sulfur bonds (41). A similar analysis reveals significant S p contribution due to delocalization of the phenoxyl hole for ½1 SR2 þ ( Table 2).…”

Redox dynamics of sulphur with Ni/GDC anode during SOFC operation at mid- and low-range temperatures: An operando S K-edge XANES study

“…High temperature oxidation and reduction studies [34] and catalysis studies [36] have also been made recently with in-situ or operando sulphur XAS, for example. Sulphur as the ligand ion is also of interest in connection with metal ions [37] and could in future studies open up new opportunities for the understanding and quantification of electronic transport processes in SOFC anodes, where newly formed NieS compounds are subject to exchange interactions, in analogy to 3d metal oxides at SOFC cathodes, for example [38,39]. Latter would constitute an extension of sulphur XAS from molecular structure and chemical speciation of sulphur motifs towards electronic structure, valence band and electronic transport properties of aged SOFC anodes.…”

Sulfur K-edge XAS of WVO vs. MoVO bis(dithiolene) complexes: Contributions of relativistic effects to electronic structure and reactivity of tungsten enzymes

“…In addition, normalization procedures can introduce ~3% error in the total pre-edge peak areas. Conversion between peak area and sulfur covalency was accomplished using the equation where D 0 is the peak area, N is the number of holes in the corresponding molecular orbital, α 2 is the sulfur covalency, and I(S) is the transition dipole integral as determined using a previously described method [22].…”