Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO₃ versus Oxyl in UO₃

Uranium trioxide, UO₃, has a T-shaped structure with bent uranyl, UO₂²⁺, coordinated by an equatorial oxo, O^2-. The structure of cation UO₃⁺ 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(NO)] (complex ), eliminates NO₂ to produce nitrate-coordinated UO₃⁺, [UO(O^•)(NO)] (), which ejects NO₃ to yield UO₃ in [UO(O)(NO)] (). Finally, associates with H₂O to afford uranyl hydroxide in [UO(OH)(NO)] (). IRMPD of , , and confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: () radical oxyl O^•-; () oxo O^2-; and () two hydroxyls, OH^-. As the nitrates are bidentate, the equatorial coordination is six in , five in , four in , and three in . Ligand congestion in low-coordinate suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in epitomizes the inverse trans influence in UO₃, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν₃ IR frequencies indicate the following donor ordering: O[best donor] ≫ O> OH> NO.