Pentavalent uranyl complexes

The uranyl dication, [UO2](2+), is the most prevalent and most thermodynamically stable form of uranium and is a soluble and problematic environmental contaminant. It is also extraordinarily chemically robust due to the strongly covalent trans-UO2 bonding. In contrast, the pentavalent uranyl cation [UO2](+) is unstable in an aqueous environment with respect to disproportionation into tetravalent uranium species and [UO2](2+). Aside from fundamental interest, an understanding of the pentavalent [UO2](+) cation is desirable since it is important environmentally as a key intermediate in the precipitation of uranium from groundwater. In the last 2 years, the use of anaerobic coordination chemistry techniques and organometallic reagents has allowed the isolation of a few kinetically inert complexes containing the f(1) [UO2](+) cation. The synthesis and characterisation of these, and the insight they give into subsequent reactivity of the trans-UO2 unit, is discussed in this review. (C) 2009 Elsevier B.V. All rights reserved.