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

Abstract

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]-(complexA1), 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 ofB1C1, andD1confirms 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 inA1, five inB1, four inD1, and three inC1. Ligand congestion in low-coordinateC1suggests orbital-directed bonding. Hydrolysis of the equatorial oxo inC1epitomizes 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-