Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO.

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, AnO(CO), where An = Pa or U, are essentially actinyl ions, AnO, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, AnO(OH)(CO). The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas Pa is inaccessible, U is very stable. The UO(CO) complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O to yield UO and two CO molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, UO, 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 PaO(CO) toward O is a manifestation of the resistance toward oxidation of Pa, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO, reacts with water to yield UOH. Infrared spectra obtained for UOH confirm the computed lowest-energy structure, UO(OH).