Reaction of {[UO2Py5][mu-KI2Py2]}(n) (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl {[UO2(dbm)(2)](2)[mu-K(Py)(2)](2)[mu(8)-K(Py)]}(2)I-2 center dot Py-2 (2) (in 70% yield) and {[UO2(dbm)(2)](2)[mu-K(MeCN)(2)][mu(8)-K]}(2) (3) (in 40% yield) in which four UP2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+center dot center dot center dot K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)(2)K(18C6)](2) (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. H-1 and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+center dot center dot center dot UO2+ and UO2+center dot center dot center dot K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)(4)] and [UO2(dbm)(2)] species. The UO2+center dot center dot center dot K+ interaction plays an important synergistic role in the stabilization of the UO2+center dot center dot center dot UO2+ interactions. Accordingly, the lower affinity of (K(18C6))(+) for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UO2+center dot center dot center dot UO2+ interaction. The UO2+center dot center dot center dot UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in X versus T at similar to 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.