Comparison of pyridine and 4-methyl-pyridine as ligands to cobalt(II) chloride in the gas phase and in solution

• Published in: Polyhedron 2001-03, Vol.20 (5), p.387-394
• Main Authors: Audergon, Laurent, Emmenegger, Franzpeter, Piccand, Michel, Piekarski, Henryk, Mokrzan, Jerzy
• Format: Article
• Language: English
• Subjects: 4-Methyl-pyridine; Cobalt(II)chloride; Gaseous metal complexes; Ligand basicity; Solvation; Thermodynamic cycle
• ISSN: 0277-5387
• DOI: 10.1016/S0277-5387(00)00635-5

In the gas phase 4-picoline is a much stronger base than pyridine (Δp K 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 2L(g) as −233 kJ mol −1 for L=4-picoline and −156 kJ mol −1 for L=pyridine. For CoCl 2L(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).