On the Nature of the Carbonyl versus Phosphoryl Binding in Uranyl Nitrate Complexes†

The electronic structure of ligands with phosphoryl and carbonyl binding sites and their complexation behavior with uranyl nitrate were investigated using density functional theory (DFT). The quantum chemical calculations indicate that the electronic charges on both phosphoryl and carbonyl groups are more polarized toward oxygen atoms in isolated ligands. This effect is predominant in the case of complexes of the former. Both P═O and C═O groups are positively charged with the exception in methylisobutylketone (MIBK), where the C=O group is virtually neutral. The fragment molecular orbital analysis suggests that during complexation, a certain amount of charge transfer occurs from the filled pπ-orbitals [π and π] of the ligand to 5f, 6d, and 7s orbitals of the uranium atom (f_σ* and ds_σ*). The NBO analysis reaffirms the charge transfer mechanism. The observed red shift in ν(C═O) and ν(P═O) identified in the simulated infrared spectrum of the corresponding complexes implies a moderate weakening of both carbonyl and phosphoryl bonds upon complexation. The atoms in molecules (AIM) analysis suggests a stronger phosphoryl binding compared to carbonyl interactions and an ionic U-O bond. The estimated complexation energies are considerable for phosphoryl ligands compared to those of the carbonyl analogue, with a reasonably large value derived for tri-n-butyl phosphate (TBP). The energy decomposition analysis marked significant stabilizing orbital interactions for phosphoryl ligands. The contributions of estimated dispersion energies are considerable in all complexes and extensively depend on the alkyl unit.