A pair of structural isomers was isolated at room temperature for the thermochromic nickel complex bis(N-isopropyl-5,6-benzosalicylideneiminato)nickel (II); one is a diamagnetic green form with square-planar coordination geometry (G phase), and the other is a paramagnetic brown form with a tetrahedral geometry (B phase). However, a question as to which form is thermodynamically stable was left open. To solve this problem, thermal and magnetic properties of this complex were investigated by adiabatic heat capacity calorimetry in the 6-508 K temperature range and magnetic measurements in the 2-400 K region. In addition to the two forms previously reported, two metastable crystal forms (G' and B' phases) were found. The stable phase sequence was G phase, B phase, and then liquid upon heating. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at around 50 K. By rapidly cooling the liquid, a glassy liquid state was realized below similar to 290 K. The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G', and then the B' phase. The entropy, enthalpy, and Gibbs energy differences, between the B and the G phases at 298.15 K were S degrees(B) - S degrees(G) = 32.8 J K-1 mol(-1), H degrees(B) - H degrees(G) = 16.0 U mol(-1), and G degrees(B) - G degrees(G) = 6.25 W mol(-1), respectively.