Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO₄^–

Abstract

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, An^VO₂(C₂O₄)^-, where An = Pa or U, are essentially actinyl ions, An^VO₂⁺, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, An^VO(OH)₂(C₂O₄)^-. The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas Pa^VI is inaccessible, U^VI is very stable. The U^VO₂(C₂O₄)^- complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O₂ to yield UO₄^- and two CO₂ molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, U^VIO₂²⁺, coordinated in the equatorial plane by two equivalent O atoms each having formal charges of -1.5 and U-O bond orders intermediate between single and double. The unreactive nature of Pa^VO₂(C₂O₄)^- toward O₂ is a manifestation of the resistance toward oxidation of Pa^V, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO₄^-, reacts with water to yield UO₅H₂^-. Infrared spectra obtained for UO₅H₂^- confirm the computed lowest-energy structure, UO₃(OH)₂^-.