The cyclic multinuclear complexes [Co-n(L-N,S,N')(n)(tacn)(n)](n+) (L = purine-6-thiones; tacn = 1,4,7-triazacyclononane) were synthesized through self-assembly of the mononuclear complex [Co(HL-N,S)Cl(tacn)](+) based on the multidentate bridging ability of purine-6-thione. Three kinds of purine-6-thione ligands were used : 9H-purine-6(1H)-thione (H(2)put), 2-amino-9H-purine-6(1H)-thione (H(2)aput), and 3,6-dihydro-6-thioxo-9H-purin-2(1H)-one (H(3)tpuo). The complexes [Co-n(L-N,S,N')(n)(tacn)(n)](n+), n = 3 and 4 for put and n = 4 for aput and Htpuo, were characterized by NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and X-ray crystal structure analysis. ESI-MS proved to be a very powerful tool for determining the nuclearity (n value) unambiguously. X-ray crystal structure analyses of [Co-4(put)(4)(tacn)(4)](CF3SO3)(4). 12H(2)O [triclinic, P (1) over bar (No. 2)] and [Co-4(aput)(4)(tacn)(4)]Cl-4. 16H(2)O [monoclinic, C2/c (No. 15)] reveal that all purinethione ligands act as tridentate ligands via S-6/N(7) chelation and the N(9) bridge and both tetranuclear complexes adopt the cyclic S-4 structure which is stabilized by four intramolecular hydrogen bonds between N(3) and H-N(tacn). The trinuclear complex [Co-3(put)(3)(tacn)(3)](3+) having the C-1 structure was optically resolved. Stereochemistry for the cyclic molecular boxes containing chiral octahedral ions is developed.