ABSTRACT: Crystals of [Co(diox)2(4-NO2-py)2] (1) and [Co(diox)2(4-CN-py)2] (2) where diox are the o-dioxolene 3,5-di-tbutylsemiquinonate(SQ•-) and/or 3,5-di-t-butylcatecholate (Cat2-) ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine,are among the few known crystals presenting both thermally-induced and photoinduced ls-[M+3(SQ•-)(Cat2-)]↔hs-[M2+(SQ•-)2] valencetautomeric interconversion (VTI). In 2, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in 1it is non-cooperative. In this work, crystals of [Co(diox)2(4-NO2-py)2]·benzene (1BZ), [Co(diox)2(4-NO2-py)2]·toluene (1TL),[Co(diox)2(4-CN-py)2]·benzene (2BZ) and [Co(diox)2(4-CN-py)2]·toluene (2TL) have been prepared and analyzed by single crystalX-ray diffraction in order to investigate how solvation modulates thermally-induced VTI. Crystallographic data was also successfullyused together with two-state equilibrium equation to estimate ΔH° and ΔS° VTI thermodynamic parameters. The solvate crystals,like the non-solvated ones, present essentially reversible thermally-induced VTI. 1TL crystal presents the same monoclinicsymmetry and the same intermolecular hydrogen-bonded network of 1 and both present a non-cooperative thermal-induced VTI.1BZ crystal has triclinic symmetry and present a cooperative VTI with a thermal hysteresis of ~30 K. In contrast to 2, thermallyinducedVTI in 2BZ and 2TL is non-cooperative despite the fact that 2, 2BZ and 2TL crystals exhibit the same monoclinic symmetryand the same intermolecular hydrogen-bonded network. In 2BZ and 2TL benzene and toluene molecules as well as thet-butyl groups of the o-dioxolene molecules convert gradually from being dynamically disordered at about 300 K to a static disorderstate below 150 K. The layer separation distance of interacting [Co(diox)2(4-X-py)2], X=CN and NO2, molecules in all solvate crystalsis ~15 Å whereas in the 2, which presents cooperative VTI, it is ~12 Å. An order-disorder component might account to the stabilizationof the metastable hs-Co2+ state in 2BZ and in 2TL but no disorder was found in the 1TL crystals. Therefore, the lack ofcooperativity in the thermally-induced VTI in these crystals seems to be due to the large distance between the layers of interactingmolecules. Cooperativity in the VTI of 1BZ crystal is likely to be related with the unique molecular bond scheme network that connectsneighboring active [Co(diox)2(4-NO2-py)2] molecules through the o-dioxolene oxygen atoms bonded directly to the Co ion.