Molecular and environmental facets of pentavalent uranium chemistry

The chemistry of uranium continues to be a burgeoning research field and has fascinated researchers for a number of decades. This is a result of its unique and diverse chemical reactivity and the fact it is the major component by mass of spent nuclear fuel from fission. Of all the available oxidation states of uranium, the pentavalent state is becoming increasingly relevant to nuclear research in natural and engineered biological and geochemical systems as well as playing a pivotal role in uranyl(VI) photoredox chemistry. Once considered a chemical curiosity, known to rapidly disproportionate to uranyl(VI) and U(IV), the +V oxidation state of uranium in the form of both uranyl(V) and uranium(V) were until recently viewed as transitory intermediates in biotic and abiotic reduction processes from its fully oxidised and environmentally prevalent form uranyl(VI). However, there is now an increasing body of evidence that uranium in the +V oxidation state can be stabilised and incorporated into biomass and iron containing minerals during reductive biological and geochemical processes over much longer timescales. In this Chapter, we discuss recent advances in understanding of the molecular chemistry and biogeochemistry of pentavalent uranium in both the dioxo and non-oxo forms, uranyl(V) and uranium(V) respectively, and the implications with respect to unravelling the chemistry of this unusual oxidation state and the remediation of uranium based nuclear activities