Ground electronic state description of thiourea coordination in homoleptic Zn²⁺, Ni²⁺ and Co²⁺ complexes using sulfur K-edge X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption spectroscopy (XAS) was employed to experimentally characterize the coordinative bond between the thio­urea (TU) or thio­carbamide ligand and transition metal (TM) ions Zn²⁺, Co²⁺ and Ni²⁺ in distorted tetrahedral and octahedral homoleptic coordination environments. Comparisons of XAS spectra of the free TU ligand and [Zn(TU)₄]²⁺, [Co(TU)₄]²⁺ and [Ni(TU)₆]²⁺ complexes clearly identify spectral features unique to TM²⁺–S(TU) bonding. Quantitative analysis of pre-edge intensities describes the covalency of Ni²⁺—S(TU) and Co²⁺—S(TU) bonding to be at most 21% and 9% as expressed by the S 3p contributions per TM 3d electron hole. Using relevant Ni²⁺ complexes with di­thio­carbamate and thio­ether ligands, we evaluated the empirical S 1s → 3p transition dipole integrals developed for S-donor ligands and their dependence on heteroatom substitutions. With the aid of density functional theory-based ground electronic state calculations, we found evidence for the need of using a transition dipole that is dependent on the presence of conjugated heteroatom (N) substitution in these S-donor ligands.