Electronic structure of [Ni(II)S4] complexes from S K-edge X-ray absorption spectroscopy

Abstract

The nickel ion has a remarkably rich coordination chemistry among the first-row transition metals. Complexes with sulfur containing ligands are particularly notable, since they can manifest classical/metal-based (innocent) or inverted/ligand-based (non-innocent) behavior depending on the chemical composition of the S-ligands and the coordination geometry. Using sulfur K-edge X-ray absorption spectroscopy (XAS), we established a spectrochemical series for [Ni(II)S4] complexes containing thiolate, aliphatic dithiolate, olefinic and aromatic enedithiolate, conjugated dithiocarbamate, and aliphatic thioether ligands. The pre-edge intensities at the sulfur K-edge follow an increasing trend from tetrathiolate through dithiolate and enedithiolate to tetrathioether. In order to obtain quantitative sulfur orbital compositions from XAS data, we generalized the earlier methods of estimating the sulfur 1s→3p transition dipole integral for a broad range of S-ligands by considering chemical shifts of spectroscopic features due to changes in the sulfur effective nuclear charge and S-ligand coordination to Ni. The XAS-based experimental orbital compositions are compared with a comprehensive set of density functional theory-based, electronic structure calculations. The 1,2-dithiolate ligands gave indication of inverted bonding, independently whether the coordinated sulfur centers are connected by constrained single or double C,C bonds. Despite intense pre-edge features, both dithiocarbamate and thioether complexes can be described with classical inorganic bonding description.