Evaluating 5f Orbital Covalency in UX6− (X = F, Cl) with U M-edge Resonant Inelastic X-ray Scattering

The nature of covalent orbital mixing between the An 5f orbitals and ligand orbitals remains an outstanding question. Actinide electronic structure is often difficult to evaluate due to large spin-orbit coupling and the presence of multiple 5f electrons (l = 3). However, this evaluation can be simplified by choosing high-symmetry systems with a low number of 5f electrons [1, 2]. Systems with a single 5f electron are particularly useful, as f-orbital energetic splitting can be modeled relatively simply using group theory [2]. In this study, the electronic structure of the f1 UX6− (X = F, Cl) molecules is studied using U M-edge resonant inelastic X-ray scattering (RIXS). With the results of core-to-core and valence band RIXS, the energetic splitting of the 5f orbitals and the delocalization of 5f electron density is evaluated [3, 4]. These results are compared to similar studies on other U(V) systems, most of which exist as the linear UO2+ moiety. This comparison helps quantify the stabilization of U(V) systems enabled by terminal oxo ligands relative to similarly electronegative halide ligands.