Anaerobic reaction of Co(O2CMe)2·4H2O with the thioether-containing acyclic pyrazine amide hexadentate ligand 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H2L11) (–CH2CH2– spacer between the two pyrazine amide tridentate coordination units) furnishes [CoII(L11)]·MeOH (1a) having CoN2(pyrazine)N′2(amide)S2(thioether) coordination. It exhibits an eight-line EPR spectrum, attesting to a low-spin (S = 1/2) state of CoII. A similar reaction in air, however, furnishes [CoIII(L3a3a)(L3b3b)]·2MeOH (2a) (S = 0), resulting from a C–S bond cleavage reaction triggered by an acetate ion as a base, having CoN2(pyrazine)N′2(amide)S(thioether)S′(thiolate) coordination. On the other hand, the reaction of Co(O2CMe)2·4H2O with 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,5-dithiopentane (H2L22) (–CH2CH2CH2– spacer between the two pyrazine amide tridentate coordination units) in air affords a cobalt(II) complex [CoII(L22)]·MeOH (1b·MeOH) (S = 1/2); its structurally characterized variety has the composition 1b·C6H6. Interestingly, 1b·MeOH undergoes facile metal-centred oxidation by aerial O2–H2O2–[Fe(η5-C5H5)2][PF6], which led to the isolation of the corresponding cobalt(III) complex [CoIII(L22)][ClO4] (2b). When treated with methanolic KOH, 2b affords a low-spin (S = 0) organocobalt(III) complex [CoIII((L2′2′)] (3). Structures of all complexes, except 1a, have been authenticated by X-ray crystallography. A five-membered chelate-ring forming ligand L11(2–) effects C–S bond cleavage and a six-membered chelate-ring forming ligand L22(2–) gives rise to Co–C bond formation, in cobalt(III)-coordinated thioether functions due to α C–H bond activation by the base. A rationale has been provided for the observed difference in the reactivity properties. The spectroscopic properties of the complexes have also been investigated. Cyclic voltammetry experiments in MeCN–CH2Cl2 reveal facile metal-centred reversible-to-quasireversible CoIV–CoIII (or a ligand-centred redox process; 2a), CoIII–CoII (1a, 1b·MeOH, 2a, 2b and 3), CoII–CoI (1a, 1b·MeOH, 2a and 2b), and CoI–Co0 (1a, 1b·MeOH and 2b) redox processes.