Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO 2py 5)(KI 2py 2)] n (1) with the Schiff base ligands salen 2-, acacen 2-, and salophen 2- (H 2salen=N,N′- ethylene-bis(salicylideneimine), H 2acacen=N,N′- ethylenebis(acetylacetoneimine), H 2salophen=N,N′-phenylene- bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen 2- in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, [UO 2(acacen)] 4[μ 8-] 2[K([18]C-6)(py)] 2 (3) and [UO 2(acacen)] 4[μ 8-]·2[K([222])(py) ] (4), [UO 2(salophen)] 4[μ 8-K] 2[μ 5-KI] 2[(K([18]C-6)]·2[K([18]C-6) (thf) 2]·2I (5), and [UO 2(salen) 4][μ 8-Rb] 2[Rb([18]C-6)] 2 (9) ([222]=[222]cryptand, py=pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U VO 2(salen)(py)][Cp 2Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K + and Rb +, whereas Li + leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U VO 2(salen)(py)][Cp 2Co] (Cp=pentamethylcyclopentadienyl) with its U VI analogue yields the oxo-functionalized dimer [UO 2(salen)(py)] 2[Cp 2Co] (8). The reaction of the [UO 2(salen) 4][μ 8-K] 2[K([18]C-6)] 2 tetramer with protons leads to disproportionation to U IV and U VI species and H 2O confirming the crucial role of the proton in the U V disproportionation. Size matters: Large alkali ions promote the assembly of Schiff base complexes of pentavalent uranyl into three new stable polynuclear cation-cation clusters, while small cations assemble unstable intermediates, resulting in disproportionation (see figure). Protons also promote disproportionation. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.