The reaction of isophthaloylbis(N,N-diethylthiourea), H2L1, with UO2(CH3COO)(2)center dot 2H(2)O and NEt3 as a supporting base gives a tetranuclear, anionic complex of the composition [{UO2(L-1}(4)(OAc)(2)](2-), in which the uranyl ions are S,O-chelate bonded. Each two of them are additionally linked by an acetato ligand. Similar reactions of various uranyl starting materials (uranyl acetate, uranyl nitrate, (NBu4)(2)[UO2Cl4]) with corresponding pyridine-centered ligands (pyridine-2,6-dicarbonylbis(N,N-dialkylthioureas), H2L2) yield mononuclear, neutral compounds, in which the thiourea derivatives are coordinated as S,N,N,N,S-five-dentate chelators. The equatorial coordination spheres of the formed hexagonal bipyramidal complexes [UO2(L-2)(solv)] are completed by solvent ligands (H2O, MeOH, or DMF). Attempted reactions without a supporting base result in decomposition of the organic ligands and the formation of hexanuclear uranyl complexes with pyridine-2,6-dicarboxylato ligands, while the use of an excess of base results in condensation and the formation of dinuclear [{UO2(L-2)(mu-OMe)}(2)](2-) complexes. A stable complex of the composition [UO2(L-3)] results from reactions of common uranyl starting materials with 2,2'-bipyridine-6,6'-dicarbonylbis(N,N-diethylthiourea) (H2L3). The equatorial coordination sphere of the neutral, hexagonal bipyramidal complex is occupied by an SN4S donor atom set, which is provided by the hexadentate organic ligand. While the uranium complexes with {L-1}(2-) and {L-2}(2-) are labile and rapidly decompose in acidic solutions, [UO2(L-3)] is stable over a wide pH range, and the ligand readily extracts uranyl ions from aqueous solutions into organic solvents.