This work is part of an effort to develop chelating agents for stable binding and easy conjugation of Re-188 to biologically interesting structures. Starting from the well-known in vivo stability of [(ReO)-Re-188(DMSA)2](-), we want to exploit this coordination system for the design of (ReO)-Re-188(V) chelates, which are stable toward reoxidation to perrhenate and toward ligand exchange under all conditions of radiopharmaceutical development. Therefore, a new type of tetradentate ligand has been synthesized by bridging two molecules of N,N-diisobutyl-2,3-dimercaptosuccinamide with N-(3-aminopropyl)propane-1,3-diamine. The resulting stereoisomeric tetrathiolato S-4 ligand of composition ((BU)-B-i)(2)N(O)C-C(SH)-C(SH)-C(O)NH-(CH2)(3)-NH-(CH2)(3)-NHC(O)-C(SH)-C(SH)-C(O)N((BU)-B-i)(2) forms anionic fivecoordinate oxorhenium(V) complexes by a ligand-exchange reaction of NBU4[ReOCl4] in methanol. In the absence of a base, the compounds were isolated as "betaine", [ReO(S-4)], with the protonated nitrogen of the bridge serving as an internal "counterion". Two representatives have been fully characterized in both the solid and solution states and found to adopt the expected square-pyramidal coordination geometry. The equatorial plane is formed by four thiolate sulfur atoms, whereas the oxygen occupies the apical position. The orientation of the metal oxo group is exo in relation to the carbamido groups in both isomers. Both complexes are stereoisomeric regarding the junction of the triamine chain.