Using solution- and solid-state S K-edge X-ray absorption spectroscopy with density functional theory to evaluate M-S bonding for MS42- (M = Cr, Mo, W) dianions

Abstract

Herein, we have evaluated relative changes in M-S electronic structure and orbital mixing in Group 6 MS42- dianions using solid- and solution-phase S K-edge X-ray absorption spectroscopy (XAS; M = Mo, W), as well as density functional theory (DFT; M = Cr, Mo, W) and time-dependent density functional theory (TDDFT) calculations. To facilitate comparison with solution measurements (conducted in acetonitrile), theoretical models included gas-phase calculations as well as those that incorporated an acetonitrile dielectric, the latter of which provided better agreement with experiment. Two pre-edge features arising from S 1s → e∗ and t∗2 electron excitations were observed in the S K-edge XAS spectra and were reasonably assigned as 1A1 → 1T2 transitions. For MoS42-, both solution-phase pre-edge peak intensities were consistent with results from the solid-state spectra. For WS42-, solution- and solid-state pre-edge peak intensities for transitions involving e∗ were equivalent, while transitions involving the t∗2 orbitals were less intense in solution. Experimental and computational results have been presented in comparison to recent analyses of MO42- dianions, which allowed M-S and M-O orbital mixing to be evaluated as the principle quantum number (n) for the metal valence d orbitals increased (3d, 4d, 5d). Overall, the M-E (E = O, S) analyses revealed distinct trends in orbital mixing. For example, as the Group 6 triad was descended, e∗ (π∗) orbital mixing remained constant in the M-S bonds, but increased appreciably for M-O interactions. For the t∗2 orbitals (σ∗ + π∗), mixing decreased slightly for M-S bonding and increased only slightly for the M-O interactions. These results suggested that the metal and ligand valence orbital energies and radial extensions delicately influenced the orbital compositions for isoelectronic ME42- (E = O, S) dianions. This journal is