Air oxidation of the six-coordinate (Et4N)2[Co(N2S2)(CN)(CH3CN)] complex derived from N,N′-(3-mercapto-3-methylbutyryl)-o-phenylenediamine results in the formation of the trinuclear mixed-valence complex (Et4N)4{CoII[CoIII{N2(SO2)2}(CN)(OH)]2} (3) where the thiolates have been oxidized to sulfinates. Its X-ray structure reveals a linear arrangement of cobalt ions with bridging hydroxides and sulfinates. All cobalt ions are in a distorted octahedral environment. The mixed-valence nature of this complex has been demonstrated by temperature-dependent magnetic susceptibility measurements and by EPR spectroscopy both in the solid state and in solution. These data are in agreement with a high-spin CoII. The IR spectrum shows the typical vibrations of cyanide and sulfinate at 2130 cm−1 (νCN), 1175 (νas/SO2) and 1030 cm−1 (νs/SO2). Trapping of CoII by chelex chromatography after treatment with aqueous HCl, or ” in situ” in aqueous solution, yields six-coordinate low-spin complexes with two different axial ligands Na3[Co{N2(SO2)2}(CN)(Cl)] (5a) and (Et4N)2[Co{N2(SO2)2}(CN)(H2O)] (5b). The pKa of the bound water molecule was estimated as 10.1 by pH-metric titration. While 3 and 5a were inactive toward acetonitrile hydration, 5b catalyzes acetamide formation at 50 °C and pH 4.7 and 7 with low efficiency. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)