U−Oyl stretching vibrations as a quantitative measure of the equatorial bond covalency in uranyl complexes:a quantum-chemical investigation

The molecular structures of a series of uranyl (UO22+) complexes in which the uranium center is equatorially coordinated by a first-row species are calculated at the density functional theory level and binding energies deduced. The resulting electronic structures are investigated using a variety of density-based analysis techniques in order to quantify the degree of covalency in the equatorial bonds. It is shown that a consideration of the properties of both the one-electron and electron-pair densities is required to understand and rationalize the variation in axial bonding effected by equatorial complexation. Strong correlations are found between density-based measures of the covalency and equatorial binding energies, implying a stabilizing effect due to covalent interaction, and it is proposed that uranyl U–Oyl stretching vibrational frequencies can serve as an experimental probe of equatorial covalency