Understanding the origins of Oyl-U-Oyl 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 degrees); 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 degrees. This Frontier Article catalogs every reported uranyl complex that features an O-yl-U-O-yl angle below 172 degrees, 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 coligands 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.