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)(2)bipy. This reaction has been studied by vis spectroscopy from 180 to 370 degrees C in the gas phase and at 25 degrees C in cyclohexane and toluene. The high temperature spectra were evaluated by linear programming yielding for the dissociation of Co(tmhd)(2)bpy(g) into Co(tmhd)(2)(g) and bpy(g) : Delta(diss)H degrees = 97 kJ mol(-1) and Delta(diss)S degrees = 128 J mol(-1) K-1. The formation of Co(tmhd)(2)bpy 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)(2)bpy is the same in cyclohexane and in toluene : Delta(assn)H degrees = -56 kJ mol(-1) and Delta(assn)S degrees = -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 enthalpy of solvation of the three equilibrium partners was measured by calorimetry and by the temperature dependence of the solubility. The complete thermodynamic cycle for this ligand addition reaction distinguishes the two contributions to the stability of the complex : the cobalt-bipyridine bond energy (97 kJ mol(-1)) and the solvation energies. The solvation energy of bpy makes the complex more stable in the gas phase than in solution.