Evaluation of Enthalpy Change and Activation Enthalpy of the Reaction of Metal Complexes in Solution

The enthalpy differences (DELTAH^0) of the unique equilibrium between octahedral and tetrahedral cobalt(II) ion have been obtained in some primary alkylamine such as n-propylamine (pa, 36.1*2.3 kJ mol^), n-hexylamine (34.9*1.0 kJ mol^), 2-methoxyethylamine (meea, 44.8*3.1 kJ mol^), and benzylamine (50.1*3.6 kJ mol^). The differences between energy levels of the six- and four-coordinate Co(II) complexes in the spherically symmetric field (DELTAE_) in the primary alkylamine solvents have been estimated from the values of deltaH^0 by offsetting the ligand field stabilization energies for the octahedral and tetrahedral geometries obtained from the absorption spectral data. It has been confirmed that the values of DELTAE_ is the decisive factor to determine the value of DELTAH^0 and is largely dependent on the electronic repulsion. This concept has been applied to the discussion about difference in energy levels between ground state and dissociative transition state of the solvent exchange reaction. The comparison between activation enthalpy (DELTAH^*) of the solvent exchange reaction of cobalt(II) ion in pa and meea has experimentally revealed that unexpectedly large rate constants of cobalt(II) ion in pa is attributed to small. DELTAH^* value which results from the strong electronic repulsion in the ground state.