Modeling Complexes of the Uranyl Ion UO₂L₂ⁿ⁺:  Binding Energies, Geometries, and Bonding Analysis

A systematic study has been carried out with density functional theory Of UO2L2n+ (n = 0 or 2) complexes. Thirty three ligands have been considered, enabling a large database of binding energies and geometries to be established that can improve our knowledge and understanding of the bonding between the uranyl ion UO22+ and potential ligands or solvent. A statistical study has been performed using the database with the twin aims of predicting the coordination energies of new complexes and revealing the dominant bonding parameters. We have shown that it is possible to predict the coordination energy satisfactorily just from the properties of the isolated ligand. However, the statistical analysis is shown to be physically unsound. To develop our understanding of the nature of the uranyl-ligand bond, the coordination energy has been decomposed into several contributions: electrostatic, repulsion, ligand polarization, uranyl polarization, charge transfer from the ligand to the uranyl ion, and inverse charge transfer. It is clear that the electrostatic term cannot be modeled just by the ligand's dipole moment; a multipole development extending at least to the quadrupole moment is necessary. Our quantitative analysis also shows that the polarization and charge-transfer terms are important and must be included in any force field if numerically and physically reliable results are sought.