The complete thermodynamic cycle of CoCl₂–pyridine complex formation

Abstract

The formation of [CoCl₂(py)] and [CoCl₂(py)₂] from CoCl₂ 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 [CoCl₂(py)] and [CoCl₂(py)₂] are much larger in the gas phase than in solution. In cyclohexanone CoCl₂ is an uncharged tetrahedral solvent complex [CoCl₂(solv)₂], which upon addition of pyridine forms [CoCl₂(py)·solv] and [CoCl₂(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 [CoCl₂(py)₂] is -231 kJ mol^-1, in cyclohexanone -52 kJ mol^-1 and in acetonitrile -49 kJ mol^-1.

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