New theoretical insights into the actual oxidation states of uranium in the solid-state compounds

Oxidation state (OS) is one of the most important concepts in chemistry and, the challenging task is how to rigorously determine the OS of multivalent solid ions of which uranium is the outstanding example. Herein, we revisit the OS of solid uranium ions in a rich variety of compounds by calculating density functional theory (DFT)-based OS (OS_DFT). The results disclose the variations of uranium OS_DFT with chemical surrounding and local structure; OS_DFT is generally consistent with formal OS (OS_f) in the ionic approximation for the majority of compounds. Interestingly, uranium OS_DFT in some compounds containing uranyl or uranyl-like units, e.g., β-/γ-UO₃, and UO₂X₂ (X = F, Cl, Br, OH), adopt U⁴⁺, instead of the textbook and never contradicted U⁶⁺ for gaseous and liquid uranyl ions. The associative effects from ligand-ligand interaction, steric repulsion, crystal-field splitting and delocalization-localization dual nature of 5f electrons are proposed to be responsible for the unusual OS behavior in solid-state uranium compounds.