The reaction of the UO2+ precursor [(UO 2Py5)(KI2Py2)]n, with the potassium salt of the tetradentate aza β-diketiminate ligand L (L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate) affords the first homometallic cation-cation complex of pentavalent uranyl. The complex [UO2L] 3 has a new triangular geometry of the cation-cation interaction in the solid state, which gives rise to a clear magnetic interaction with a maximum in the plot of χ versus T at 12 K. It retains its solid state trinuclear structure in solution and is fully stable in organic anaerobic solvents, but reacts rapidly with molecular oxygen to form a rare dinuclear oxo complex of uranyl(vi), ([UO2(L)]22-O]). © 2012 The Royal Society of Chemistry.