On the “yl” bond weakening in uranyl(VI) coordination complexes

Abstract

The U–O_yl triple bonds in the UO₂²⁺ aquo ion are known to be weakened by replacing the first shell water with organic or inorganic ligands. Weakening of the U–O_yl bond may enhance the reactivity of “yl” oxygens and uranyl(VI) cation–cation interactions. Density functional theory calculations as well as previously published vibrational spectroscopic data have been used to study the origin of the U–O_yl bond weakening in uranyl(VI) coordination complexes. Natural population analyses (NPA) revealed that the electron localization on the O_yl 2p orbital is a direct measure of the U–O_yl bond weakening, indicating that the bond weakening is correlated to the weakening of the U–O_yl covalent bond and not that of the ionic bond. The Mulliken analysis gives poor results for uranium to ligand electron partitioning and is thus unreliable. Further analyses of molecular orbitals near the highest occupied molecular orbital (HOMO) show that both the σ and π donating abilities of the ligands may account for the U–O_yl bond weakening. The mechanism of the bond weakening varies with coordinating ligand so that each case needs to be examined independently.