The vitality of uranium molecular chemistry at the dawn of the XXIst century.

The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000-2006 period. This discipline is currently witnessing an impressive development, together with the theoretical chemistry and solid-state chemistry of the f-elements, and its face has profoundly changed, revealing unsuspected structural and reactivity features. This progress required and was facilitated by the use of new precursors. Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation and led to the discovery of unusual reactions, including activation of small molecules. A number of tetravalent uranium complexes, in particular polynuclear compounds, have been synthesized, which exhibit exciting structures and physicochemical properties. The potential of uranium(III) and uranium(IV) complexes in catalysis has been confirmed. The uranyl complexes, from mononuclear species to supramolecular assemblies, reveal a variety of novel structures, changing the generally accepted ideas on the coordination geometry and the stability of the UO2(2+) ion.