The coordination ion with a well-characterized carbon-cobalt(III) bond, the (1,4,7-triazacyclononane)(1,6-diamino-3-thia-4-hexanido)cobalt(III) dication, [Co(tacn)(C-aeaps)](2+) (aeaps, for aminoethylaminopropylsulfide), has been reacted with iodomethane, and the S-methyl thionium derivative has been isolated. The crystal structure of the resulting [Co(tacn)(C-aeaps-SCH(3))]Br(3) x 3 H(2)O at 122 K has been determined by X-ray diffraction techniques to verify the structure. The crystal structure determination shows that the carbon-cobalt bond length is even shorter (2.001(4) A) than in [Co(aeaps)(C-aeaps)](2+) (2.026(3) A), while its trans elongating effect is less pronounced. The (1,4,7-triazacyclononane)(1,6-diamino-3-thia-4-hexanido)cobalt(III) dication [Co(tacn)(C-aeaps)](2+) (aeaps, for aminoethylaminopropylsulfide) reacts relatively fast with acid, for example, with NH(4)(+) to form a sulfur-bound aeaps ligand. The [Co(tacn)(C-aeaps-SCH(3))](3+) ion is remarkably robust in strongly acidic aqueous solution in spite of the supposed high basicity of the carbon anion. However, with a large excess of iodide, the methyl group can be removed as iodomethane. The experimentally obtained distances around cobalt(III) for the three involved coordination ions are compared to those computed from DFT with different standard choices for functionals and basis sets. The agreements range from poor to modest depending of the choice of functionals. It is noteworthy, however, that a sulfur 3p orbital in [Co(tacn)(C-aeaps)](2+) participates in bonding to cobalt(III), having implications for the transformation between the carbon- and sulfur-bound forms of the aeaps ligand.