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 center dot-. 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, [UO₂ (NO₃)₃]^- (complex A1), eliminates NO₂ to produce nitrate-coordinated UO₃⁺, [UO₂ (O^. )(NO₃)₂]^-(B1), which ejects NO₃ to yield UO₃ in [UO₂ (O)(NO₃)]^- (C1). Finally, C1 associates with H₂O to afford uranyl hydroxide in [UO₂(OH)₂ (NO₃)]^- (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 UO₃, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl v₃ IR frequencies indicate the following donor ordering: O^2- [best donor] >> O^.- > OH^-> NO₃^-.