Energy-Degeneracy-Driven Covalency in Actinide Bonding

Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An-Cl covalency in AnCl₆^2- (An^IV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and An^IV 5f- and 6d-orbitals (t_1u*/t_2u* and t_{2 g}*/e_g *), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amount of M-Cl orbital mixing, such that An^IV 6d - and Cl 3p-mixing decreased and metal 5f - and Cl 3p-orbital mixing increased across this series.