Uranium cations: generation

The first transition metal cation which is unstable in water but which can be generated as a stable entity in HF was U3+ [30]. It was formed by oxidation of the metal by protons in a BF3-HF solution which is non-oxidising and relatively weakly acidic. The UV-vis spectrum of the lilac-colored solution was virtually identical with that observed for an acidified aqueous solution in which the uranium solution was under continuous electrolytic reduction to maintain U(III) as the aquo-cation. 

Chapter 2 considers the removal of inorganic water contaminants using photocatalysis. Metal cations react via one-electron steps first leading to unstable chemical intermediates, and later to stable species. Three possible mechanisms are identified (a) direct reduction via photo-generated conduction band electrons, (b) indirect reduction by intermediates generated from electron donors, and (c) oxidative removal by electron holes or hydroxyl radicals. The provided examples show the significance of these mechanisms for the removal of water contaminants such as chromium, mercury, lead, uranium, and arsenic. 

A cationic bis(pentamethylcyclopentadienyl)uranium(lll) complex has been reported. [Cp 2U(THF)2][BPli4] is generated by protonation of the complex Cp 2U[N(SiMe3)2] with [NH4][BPh4]. ... 

In one of the best known industrial solvent extraction processes, uranium is leached from its ore by dilute sulfuric acid to yield the uranium(VI) species [1102(804)3] in the solution. Addition of the tertiary trioctylamine, ALAMINE 336, to this acidic solution first forms the cationic species [HN(CgHi7)3] ", which in turn generates the neutral ion-pair [U02(S04)3] [HN(CgHi7)3] J that is extracted into kerosene (Figure 10). The structure of the complex uranium-containing anion is typical of complexes formed by the uranyl species a linear UO2 group with six O atoms coordinated to the U atom around the equator. This geometry is found in several other extractable species of uranium(VI). 

The second generation Sohio catalyst was a uranium antimonate (USbsOio). This was more active and selective than the earher bismuth phos-phomolybdate and has been described as Phase I. Active sites in the layer strac-ture were also defect-Scheelite structures containing uranium-antimoity cation pairs. Catalysts containing USbOs, or Phase 2, were less selective. 

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