Mechanistic and electronic structure insights into formate formation in uranium and related actinide complexes

The stepwise formation of actinide formate complexes provides an important framework for understanding actinide-ligand bonding and reactivity. Density functional theory calculations are used to investigate the thermodynamics, kinetics, and electronic structure of formate formation reactions. On the basis of an established uranium-centered mechanism, the effects of halogen substitution (F, Cl, and Br) are examined, followed by a comparative study in which uranium is replaced with thorium, neptunium, and plutonium. The reaction and activation free energies are evaluated for all the elementary steps leading to mono-, bis-, and tris-formate intermediates (IV, VII, and X). The results reveal clear halogen- and actinide-dependent trends in intermediate stability and activation barriers. Complementary noncovalent interaction (NCI), reduced density gradient (RDG), electron localization function (ELF), and localized orbital locator (LOL) analyses reveal that the reaction pathways are governed by localized metal-ligand interactions, providing a unified mechanistic and electronic structure description across the studied actinide series.