Understanding the origins of O-U-O bending in the uranyl (UO) ion.

The structure of the uranyl ion (UO) has been the topic of investigation for almost a century. Since the first structural study of uranyl in 1935, over 4000 uranyl complexes have been characterized by X-ray crystallography. The vast majority of these complexes feature a linear uranyl group (e.g., O-U-O = 180°); however, there are a handful of complexes that feature much more acute O-U-O angles. In fact, the smallest experimentally observed O-U-O angles are ca. 161°. This Frontier Article catalogs every reported uranyl complex that features an O-U-O angle below 172°, and attempts to rationalize the origins of the observed O-U-O bending. In particular, I describe two distinct causes of O-U-O bending: (1) bending that occurs as a result of unfavourable steric interactions between the equatorial co-ligands and the uranyl oxo groups; and (2) bending that appears to have an electronic origin. In addition, I describe several possible avenues for future investigation. Understanding the effect that O-U-O bending has on uranyl electronic structure could ultimately provide insight into several unique aspects of the uranyl ion, such as the inverse trans influence and the involvement of the "pseudo-core" U6p orbitals in U-O bonding.