The complete thermodynamic cycle of CoCl2–pyridine complex formation

Abstract

The formation of [CoCl ₂ (py)] and [CoCl ₂ (py) ₂ ] from CoCl ₂ and pyridine is one of the rare cases where the formation of a complex can be studied in the gas phase and in solution. By measuring the solvation enthalpies of all the partners of the reaction a complete thermodynamic cycle for this complex-formation reaction has been established. It shows that the stabilities of [CoCl ₂ (py)] and [CoCl ₂ (py) ₂ ] are much larger in the gas phase than in solution. In cyclohexanone CoCl ₂ is an uncharged tetrahedral solvent complex [CoCl ₂ (solv) ₂ ], which upon addition of pyridine forms [CoCl ₂ (py)·solv] and [CoCl ₂ (py) ₂ ]. In acetonitrile the situation is similar but the electrical conductivity suggests some ionic dissociation. In spite of the different donor atoms the two solvents attenuate the gas-phase stability by almost the same amount; in the gas the enthalpy of formation of [CoCl ₂ (py) ₂ ] is -231 kJ mol ^-1 , in cyclohexanone -52 kJ mol ^-1 and in acetonitrile -49 kJ mol ^-1 .