PNNL

Abstract

Metallo dithiolene complexes with biological and catalytic relevance are well-known for having strong metal-ligand covalency, which dictates their valence electronic structures. We present the resonant sulfur K? (1s3p) x-ray emission spectroscopy for a series of Ni and Cu bis(dithiolene) complexes to reveal the ligand sulfur contributions to both the occupied and unoccupied valence orbitals. While S K-edge x-ray absorption spectroscopy played a critical role in identifying the covalency of the unoccupied orbitals of metal dithiolenes, the present focus on X-ray emission spectroscopy explores the occupied density of states. For a series of [Cu(mnt)2]n- and [Ni(mnt)2]n- anions and dianions, a comparison of the non-resonant and resonant S K? XES spectra highlights the dramatic improvement in spectral resolution and corresponding ability to differentiate subtle changes in occupied electronic structure across the series. Furthermore, the use of resonant inelastic x-ray scattering (RIXS) probes the valence excited states and the core-valence couplings of the complexes. By employing a theoretical approach based on time-dependent density functional theory to interpret the RIXS spectra, we reveal how metal-ligand covalency influences the excited state energies and covalencies. We identify the low energy excited states as having the same symmetry as the nominal ‘ligand field’ or ‘d-d’ states that typically dominate the photophysics of 3d metal complexes, but with significant metal-ligand charge transfer character dictated by their covalency. Overall, the results emphasize the enhanced utility of S K? X-ray emission spectroscopy of metal-sulfur complexes when measured with resonant excitation and suggest that strong metal-ligand covalency can be used to influence the charge-transfer photochemistry of first row transition metal complexes.