The Co^III complexes of the hexadentate tripodal ligands HOsen (3-(2´-aminoethylamino)-2,2-bis((2´´-aminoethylamino)methyl)propan-1-ol) and HOten (3-(2´-aminoethylthia)-2,2-bis((2´´-aminoethylthia)methyl)propan-1-ol) have been synthesized and fully characterized. The crystal structures of [Co(HOsen)]Cl₃∙H₂O and [Co(HOten)](ClO₄)Cl₂ are reported and in both cases the ligands coordinate as tripodal hexadentate N₆ and N₃S₃ donors, respectively. Cyclic voltammetry of the N₃S₃ coordinated complex [Co(HOten)]³⁺ is complicated and electrode dependent. On a Pt working electrode an irreversible Co^III/II couple (formal potential –157 mV versus Ag|AgCl) is seen, which is indicative of dissociation of the divalent complex formed at the electrode. The free HOten released by the dissociation of [Co(HOten)]²⁺ can be recaptured by Hg as shown by cyclic voltammetry experiments on a static Hg drop electrode (or in the presence of Hg²⁺ ions), which leads to the formation of an electroactive Hg^II complex of the N₃S₃ ligand (formal potential +60 mV versus Ag|AgCl). This behaviour is in contrast to the facile and totally reversible voltammetry of the hexaamine complex [Co(HOsen)]³⁺ (formal potential (Co^III/II) –519 mV versus Ag|AgCl), which is uncomplicated by any coupled chemical reactions. A kinetic and thermodynamic analysis of the [Co(HOten)]²⁺/[Hg(HOten)]²⁺ system is presented on the basis of digital simulation of the experimental voltammetric data.