Recent spectroscopic data suggest that the Co(III) site in Co-containing nitrile hydratase is ligated to carboxamido nitrogens and thiolato sulfurs and most possibly one or more of the bound thiolates exist as sulfenato and/or sulfinato groups. The absence of any Co(III) complex with such coordination makes it quite difficult to predict the reactivity of this kind of Co(III) site. In this paper, the Co(III) complexes of two designed ligands PyPepSH(2) (1) and PyPepRSH(2) (2) have been reported. The two complexes, namely, (Et4N)[Co(PyPepS)(2)] (3) and Na[Co(PyPepRS)2] (4) are the first examples of Co(III) complexes with carboxamido nitrogens and thiolato sulfurs as donors. The average Co(III)-N-amido and Co(III)-S distances in these complexes lie in the range 1.90-1.92 and 2.22-2.24 Angstrom, respectively. Reaction of H2O2 With both complexes readily affords Na[Co(PyPepSO(2))(2)] (5) and Na[Co(PyPepRSO(2))(2)] (6), species in which the thiolato sulfurs are converted to sulfinato (SO2) groups. Such conversion also occurs when solutions of 3 and 4 are exposed to dioxygen in the presence of activated charcoal, These reactions are clean and the S -> SO2 transformation does not introduce significant changes in the metric parameters of these complexes. The reactivity of 3 and 4 indicates that the bound Cys-sulfurs around the biological Co(III) site could be oxidized to sulfinato groups.