Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2−, where X = F, Cl, Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ab initio ligand field calculations, we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f inter-electron repulsion and 4f-5f spin-exchange caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.