Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO 3 versus Oxyl in UO 3 .

Uranium trioxide, UO ₃ , has a T-shaped structure with bent uranyl, UO ₂ ²⁺ , coordinated by an equatorial oxo, O ^2- . The structure of cation UO ₃ ⁺ is similar but with an equatorial oxyl, O ^•- . Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO ₂ (NO ₃ ) ₃ ] ^- (complex A1 ), eliminates NO ₂ to produce nitrate-coordinated UO ₃ ⁺ , [UO ₂ (O ^• )(NO ₃ ) ₂ ] ^- ( B1 ), which ejects NO ₃ to yield UO ₃ in [UO ₂ (O)(NO ₃ )] ^- ( C1 ). Finally, C1 associates with H ₂ O to afford uranyl hydroxide in [UO ₂ (OH) ₂ (NO ₃ )] ^- ( D1 ). IRMPD of B1 , C1 , and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: ( B1 ) radical oxyl O ^•- ; ( C1 ) oxo O ^2- ; and ( D1 ) two hydroxyls, OH ^- . As the nitrates are bidentate, the equatorial coordination is six in A1 , five in B1 , four in D1 , and three in C1 . Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO ₃ , which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν ₃ IR frequencies indicate the following donor ordering: O ^2- [best donor] ≫ O ^•- > OH ^- > NO ₃ ^- .