Understanding the origins of O_yl-U-O_yl bending in the uranyl (UO₂²⁺) ion

The structure of the uranyl ion (UO₂²⁺) 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_yl-U-O_yl = 180°); however, there are a handful of complexes that feature much more acute O_yl-U-O_yl angles. In fact, the smallest experimentally observed O_yl-U-O_yl angles are ca. 161°. This Frontier Article catalogs every reported uranyl complex that features an O_yl-U-O_yl angle below 172°, and attempts to rationalize the origins of the observed O_yl-U-O_yl bending. In particular, I describe two distinct causes of O_yl-U-O_yl 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_yl-U-O_yl 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_yl bonding.