Exploring phosphoryl oxygen basicity in U(VI) complexation: A comparative study from trialkyl phosphate to phosphine oxide

The conventional argument that extraction efficiency depends on the "basicity of the phosphoryl oxygen" is thoroughly examined in this study. The analysis involves studying the electronic structures of various ligands, such as phosphate, phosphonate, phosphinate, and phosphine oxide, as well as variations in their alkyl chain length, and their corresponding uranium complexes. The studies revealed a significant amount of destabilizing strain and steric repulsion for ligands having longer alkyl chains upon complexation. A considerable amount of stabilizing orbital and dispersion interactions compensate for these repulsions, forming stable complexes. Dispersion interactions become more significant upon chain elongation and are mainly responsible for the preference for U(VI) metal ions by ligands with lengthy alkyl chain units. The preference of phosphine oxide ligands for U(VI) is analyzed within the context of enhanced orbital interactions resulting from the energetically close donor (ligand) and acceptor (metal nitrate) orbitals. Additionally, dispersion-based interactions also become significant, especially with larger chain lengths. The electronegative environment around the phosphorus atom, along with the existence of low-dipole moment structures, is also examined in relation to their possible role in solvent extraction and their influence on the selectivity of ligands for uranyl species.