In the gas phase 4-picoline is a much stronger base than pyridine (ΔpK B =2.9) and consequently it forms much stronger complexes with CoCl 2 . Using high temperature visible absorption spectroscopy we determined the enthalpy of the gas phase reaction CoCl 2 (g)+L(g)=CoCl 2 L(g) as -233 kJ mol - 1 for L=4-picoline and -156 kJ mol - 1 for L=pyridine. For CoCl 2 L(g)+L=CoCl 2 (L) 2 (g) the enthalpies are -87 kJ mol - 1 for L=4-picoline and -76 kJ mol - 1 for L=pyridine. These enthalpies correspond to the Co N bond energies, which makes them valuable for bond theory calculations. In solution (cyclo-hexanone, acetonitrile) the difference of basicity of the two ligands is attenuated and so is the difference of the stability of the complexes with CoCl 2 . For both ligands in both solvents the enthalpy of the addition of two ligands to CoCl 2 is around -50kJ mol - 1 . By measuring the solvation enthalpies of all the partners of the reaction complete thermodynamic cycles for these complex-formation reactions have been established. They show that CoCl 2 (py) 2 and CoCl 2 (pic) 2 are less stable in solution than in the gas phase because the solvation enthalpies of the reactants (~-340 kJ mol - 1 ) are much more negative than the solvation enthalpies of the products (~-150 kJ mol - 1 for CoCl 2 (py) 2 and ~-100 kJ mol - 1 for CoCl 2 (pic) 2 ).