Pressure-Induced Enhanced Covalency Difference in the Coordination of Am(III)/Eu(III) with an Unsymmetric Phenanthroline Extractant.

The influence of high pressure on the coordination properties of f-block compounds is a topic of growing interest, which provides potential applications in the separation of actinide (An) and lanthanide (Ln) ions. In this study, we explore the effect of high pressure on the bonding characteristics of the unsymmetric phenanthroline extractant (Et-Tol-CyMe4-ATPhen, ) in its complexes with Am and Eu using density functional theory (DFT) and various bonding analyses. Pressures ranging from 0 to 10 GPa are applied to investigate changes in the geometry, electronic structure, and bond covalency. Our results show that high pressure significantly enhances the covalent character of the bonds between and Am/Eu, evidenced by increased electron density ρ(r), delocalization indices (DI), stronger covalent orbital interactions (Δ), and interatomic exchange interactions (). Notably, while the Eu- bond lengths contract more than the Am- bonds, the 5f orbitals of Am are more sensitive to pressure than the relatively contracted 4f orbitals of Eu, resulting in an enhanced covalency difference confirmed by natural localized molecular orbital (NLMO) analysis. These findings highlight the significant role of high pressure in altering the bonding characteristics of f-block compounds, opening new avenues for further studies of highly efficient Am/Eu separation.