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 (An³⁺) and lanthanide (Ln³⁺) ions. In this study, we explore the effect of high pressure on the bonding characteristics of the unsymmetric phenanthroline extractant (Et-Tol-CyMe4-ATPhen, <b>L</b>) in its complexes with Am³⁺ and Eu³⁺ 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 <b>L</b> and Am³⁺/Eu³⁺, evidenced by increased electron density ρ(r), delocalization indices (DI), stronger covalent orbital interactions (Δ<i>E</i>_orb), and interatomic exchange interactions (<i>V</i>_xc). Notably, while the Eu-<b>L</b> bond lengths contract more than the Am-<b>L</b> bonds, the 5f orbitals of Am³⁺ are more sensitive to pressure than the relatively contracted 4f orbitals of Eu³⁺, 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 Am³⁺/Eu³⁺ separation.