The electronic structure of various complexes of pentavalent uranyl species, namely UO2 +, is described, using DFT methods, with the aim of understanding how the structure of the ligands may influence the localisation of the unpaired 5f electron of uranium (V) and, finally, the stability of such complexes towards oxidation. Six complexes have been inspected: [UO2py5]+ (1), [(UO 2py5)KI2] (2), [UO2(salan- tBu2)(py)K] (3), [UO2(salophen-t- Bu2)(thf)K] (4), [UO2(salen-tBu 2)(py)K] (5), [and UO2-cyclopyrrole]1- (6), chosen to explore various ligands. In the five first complexes, the UO 2 + species is well identified with the unpaired electron localized on the 5f uranium orbital. Additionally, for the salan, salen and salophen ligands, some covalent interactions have been observed, resulting from the presence of both donor and acceptor binding sites. In contrast, the last complex is best described by a UO2 2+ uranyl (VI) coordinated by the anionic radical cyclopyrrole, the highly delocalized φ orbitals set stabilizing the radical behaviour of this ligand. © 2010 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.