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 ...