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

Abstract

Uranium trioxide, UO3, has a T-shaped structure with bent uranyl, UO2 2+ , 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, [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 ν3 IR frequencies indicate the following donor ordering: O 2-[best donor] >> O •-> OH -> NO3 -.

Dinuclear Complexes of Uranyl, Neptunyl, and Plutonyl: Structures and Oxidation States Revealed by Experiment and Theory

“…Bond orders and oxidation states are inferred from An–O and O–O distances. Short An–O bonds (∼1.7–1.8 Å; e.g., a and b in G ) are assigned as AnO oxo, whereas longer bonds (∼2.1–2.2 Å; e.g., b ″ in A for Pu:Pu) are An–O • oxyl with similar distances to hydroxide single bonds, An–OH. , The O–O distance in free O 2 , 1.21 Å, increases to ca. 1.2–1.3 Å in superoxide O 2 – , and to ca.…”