In this study, a new tetrathiafulvalene-functionalized phenanthroline ligand (TTF-phen) and a mononuclear cobalt(II) complex with a redox active 3,6-di-tert-butylsemiquinonato (3,6-dtsq) ligand was synthesized. Magnetic measurements and IR spectroscopy suggested that the Co complex [Co(3,6-dtsq)(2)(TTF-phen)] exhibited thermally induced valence tautomerism in the temperature range 130-400 K. This complex is the first example of a valence tautomeric complex with a TTF-functionalized ligand.