Addition of 2,2‘-bipyridine to the red tetrahedral complex Co(tmhd)2 (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) produces the orange octahedral complex Co(tmhd)2bipy. This reaction has been studied by vis spectroscopy from 180 to 370 °C in the gas phase and at 25 °C in cyclohexane and toluene. The high temperature spectra were evaluated by linear programming yielding for the dissociation of Co(tmhd)2bpy(g) into Co(tmhd)2(g) and bpy(g): ΔdissH° = 97 kJ mol-1 and ΔdissS° = 128 J mol-1 K-1. The formation of Co(tmhd)2bpy in the noncoordinating solvents cyclohexane and toluene was studied by vis spectroscopy and by enthalpometric titration. Within the limits of error the stability of Co(tmhd)2bpy is the same in cyclohexane and in toluene: ΔassnH° = −56 kJ mol-1 and ΔassnS° = −87 J mol-1 K-1. The identical stability in the two solvents is due to an accidental compensation of the different solvation energies of the reaction partners and not to a dominant role of the cobalt−bipyridine bond energy. The entha...