The Role of Halides in the Bonding and Electronic Structure of Actinyl(VI) Halides─Energy Match Driven Stability

The role of the equatorial ligands and their influence on the electronic structures and bonding properties of uranyl and other actinyls are not well understood and are thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F^- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl^- and uranyl-Br^-. The good energy match between uranyl(VI) and F^- likely enhances the stability of the uranyl-F^- bond, contributing to the higher U-F^- affinity in aqueous solution compared to uranyl-Cl^-/Br^-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides: NaRb₈(UO₂)₅F₁₉·2H₂O, Rb₂UO₂Cl₄·2H₂O, and Rb₂UO₂Br₄·2H₂O. We describe covalent uranium-halide interactions, following the trend Br^- ≈ Cl^- > F^-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M₄ edge HR-XANES and RIXS exhibit unexpected σ* peak trends not directly correlated with U═O bond lengths but well explained by LFDFT RIXS calculations.