We successfully synthesized a room-temperature ionic liquid showing valence-tautomeric behavior, by combining a cobalt bis(dioxolene) complex dianion with a bulky trihexyl(tetradecyl)phosphonium cation. Its magnetic moment exhibits a gradual increase up to ca. 280 K through the glass transition (262 K) and then has an abrupt upturn at around 300 K. The valence-tautomeric behavior was confirmed by the temperature dependence of the electronic absorption spectra. This successful implementation allowed the "smart" ionic liquid, whose magnetic behavior responds evidently to temperature change.