Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3versus Oxyl in UO3+

Uranium trioxide, UO3, has a T-shaped structure with bent uranyl, UO22+, coordinated by an equatorial oxo, O2–. 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, [UO2(NO3)3]–(complex A1), eliminates NO2to produce nitrate-coordinated UO3+, [UO2(O•)(NO3)2]–(B1), which ejects NO3to yield UO3in [UO2(O)(NO3)]–(C1). Finally, C1associates with H2O to afford uranyl hydroxide in [UO2(OH)2(NO3)]–(D1). IRMPD of B1, C1, and D1confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: (B1) radical oxyl O•–; (C1) oxo O2–; 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 C1suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1epitomizes the inverse trans influence in UO3, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν3IR frequencies indicate the following donor ordering: O2–[best donor] ≫ O•–> OH–> NO3–.