The complexation and bonding nature of Am(III), Cm(III), and Eu(III) ions with 6 N-donor ligands of different dentate numbers were theoretically investigated. The NBO analysis and QTAIM analysis revealed that the Am metal can form more covalent with the ligands than Cm and Eu. Thermodynamic analysis demonstrated effective separation of Am from Cm and Eu, with L₄ showing superior performance for Am(III)/Cm(III) separation and L₆ excelling in Am(III)/Eu(III) separation.

Abstract

The separation of actinides (An) from lanthanides (Ln) is crucial for nuclear resource recovery and reducing long-term radiotoxicity. While numerous N-donor ligands have been examined for the extraction separation of An and Ln, few studies have investigated the effect of the number of ligand dentate on bonding and extraction separation. To address this issue, we designed six pyridine-based ligands with different numbers of coordination dentate. We employed density functional methods to investigate their bonding properties with Am(III), Cm(III), and Eu(III), as well as the thermodynamic differences in extracting and separating these metal ions. The NBO and QTAIM analyses indicate that the metal–ligand bonds are predominantly ionic. However, the Am-N bond exhibits higher covalency compared to the Cm-N and Eu-N bonds, and it is also stronger than the latter two. This difference in bonding can be attributed to the greater involvement of the 5f orbitals of Am in coordination with the ligands, in contrast to the involvement of the Cm 5f and Eu 4f orbitals. Thermodynamic analysis reveals that the coordination ability of the ligands with metals does increase with the number of coordination teeth. However, the extraction separation effect of the ligand for Am/Cm and Am/Eu does not show a strong correlation with the number of coordination dentate. We hope that this study can offer valuable theoretical support for the design of ligands aimed at the separation of actinide(III) and lanthanide(III) ions.