Trivalent actinides (An) and lanthanides (Ln) have similar chemistries, but their complexation behaviors are driven by differences in covalency, which control the ligand selectivity in liquid-liquid extraction. Actinides prefer soft-donor ligands due to an increased covalent character of the metal-ligand bond. Lanthanides, on the other hand, tend to form stronger complexes with hard-donor ligands due to an increased ionic character of the metal-ligand bond. Density functional theory and natural bond orbital methods have been employed to characterize the electronic structure of d and f orbitals in various An(III) and Ln(III) complexes. Results suggest that small differences in An/Ln-ligand bonding can account for metal-ligand bond covalency and ligand selectivity. The extent of f and d orbitals participation has been estimated with density functional theory and energy calculations were performed to support covalency findings.

Covalency differences in actinide/lanthanide complexation

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