The uranyl cation is the most environmentally relevant form of uranium and is often found in close contact with a variety of heavy metals due to its role in the nuclear fuel cycle. As such, an understanding of how these metals interact with and are able to affect the properties of the uranyl cation is of utmost importance for the purposes of nuclear forensics and nuclear waste stewardship. The research presented herein focuses on the interactions between the uranyl oxo group and two closed shell metal cations, Pb2+ and Ag+, in a large family of compounds. The structures of these compounds were determined using single crystal X-ray diffraction and their properties were probed using spectroscopic and computational measurements. From these techniques, we show that close interaction between the oxo group and the metal cation can weaken the U=O bonds as evidenced by a red-shift in the Raman active UO22+ symmetric stretch peak. This observation was rationalized by computations performed using Natural Bond Orbital analysis which indicated a depopulation of U=O bonding orbitals and a population of U=O anti-bonding orbitals as a result of strong interaction with orbitals on the metal. The stabilization energy associated with these interactions increases as the metal approaches the uranyl-oxo. Close interactions with Pb2+ also quench the characteristic uranyl luminescence, likely due to a weakening of the U=O bonds leading to a disruption of the electronic structure of the uranyl unit, though the exact mechanism remains unclear. The effect of the Ag+ on the U=O bond is comparatively weaker than that of the Pb2+, where only the two compounds with the closest metal-oxo interaction displayed a loss in luminescence. Additionally, interaction with the Ag+ results in smaller Raman shifts and smaller stabilization energy values at similar metal-oxo interaction distances. Quantum Theory of Atoms in Molecules analysis indicates a loss of electron density in the U=O bonds where the oxo groups interact closely with the Pb2+ but did not indicate a similar loss with Ag+. The difference in the effects of these two metals likely occurs as a result of the lower charge density on the Ag+ though it is also possible that other properties of the metal cations such as size or the presence (or absence) of a stereochemically active lone pair also play a role.

Effects of closed-shell metal-oxo interactions on uranyl bonding and spectroscopy

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