The new ligand 6-mercapto-2(1H)-pyridone (H2PySO) has been prepared in good yield by reaction of 6-chloro-pyridin-2-ol with NaSH. Reaction of the salt K2PySO, generated in situ, with the appropriate complex [M(mu-Cl)(diolefin)](2) affords the tetranuclear complexes [M-4(mu-PySO)(2)(diolefin)(4)] [M = Rh, diolefin = 1,5-cyclooctadiene (cod) (1), tetrafluorobenzobarralene (tfbb) (2); M = Ir, diolefin = cod (3)]. The molecular structure of complex 1 has been determined by X-ray diffraction methods. The tetranuclear structure is supported by two S,N,O-tridentate ligands exhibiting a 1 kappa O, 2 kappa N, 3:4 kappa S-2 coordination mode. Carbonylation of the rhodium diolefin complexes at atmospheric pressure gives [Rh-4(mu-PySO)(2)(CO)(8)] (4). The carbonylation of 1 is partially reversible, and the mixed-ligand complex [Rh-4(mu-PySO)(2)(cod)(2)(CO)(4)] (5) has been obtained as a single isomer. The reaction of 4 with triphenylphosphine gives the compound [Rh-4(mu-PySO)(2)(CO)(4)(PPh3)(4)] (6) which also exists as a single isomer of C symmetry. The diolefin complexes are redox active and exhibit two one-electron oxidations at a platinum disk electrode in dichloromethane separated by approximately 0.5 V at potentials accessible by chemical oxidants. The tetranuclear complexes were selectively oxidized to the 63-electron mixed-valence cationic complexes [M-4(mu-PySO)(2)(diolefin)(4)](+) (1a(+), 2(+), and 3(+)) by using AgCF3SO3 as oxidant and isolated as the triflate salts. Alternatively, the oxidation with [Cp2Fe]PF6 gives [Rh-4(mu-PySO)(2)(cod)(4)][PF6] (1b(+)). The parameters obtained from the simulation of the electron paramagnetic resonance spectra of the oxidized species strongly suggest that the unpaired electron is delocalized over only two metal atoms in the complexes.