The actinide—fluorine bond is of particular interest due to the role of actinide fluorides in the nuclear fuel cycle. Moreover, the covalent character of these bonds is of fundamental interest to actinide bonding. Ligand K-edge X-ray Absorption Spectroscopy (XAS) probes the transitions of ligand 1s orbitals into metal-based molecular orbitals and has been established as direct probe of metal—ligand covalency. In this work, Fluorine K-edge X-ray XAS is used to evaluate bond covalency in MF₆^2- (M = Ti, Zr, Hf, Ce, U, Np, Pu). Spectral assignments are guided by TD-DFT spectral simulations and ground state electronic structure calculations. For all complexes, transitions into d-based orbitals of t_2g symmetry are observed. Examining CeF₆^2-, a large pre-edge feature is observed corresponding with transitions into 4f orbitals. Finally, pre-edge features associated with transitions into 5forbitals are observed in all AnF₆^2- (An = U, Np, Pu). In UF₆^2-, a single pre-edge feature is observed as a shoulder on the rising edge. For NpF₆^2- and PuF₆^2-, multiple pre-edge features associated with transitions into the t_1u and t_2u f-orbital sets are observed. The intensities of these features are used to analyze the bonding in these molecules within a perturbational molecular orbital framework.

Dumitru-Claudiu Sergentu

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