Octahedral-tetrahedral equilibrium and solvent exchange of cobalt(II) ions in primary alkylamines

The enthalpy differences (ΔH°) of the equilibrium between the octahedral and tetrahedral solvated cobalt(II) complexes were obtained in some primary alkylamines such as propylamine (pa, 36.1 ± 2.3 kJ mol^-1), n-hexylamine (ha, 34.9 ± 1.0 kJ mol^-1), 2-methoxyethylamine (meea, 44.8 ± 3.1 kJ mol^-1), and benzylamine (ba, 50.1 ± 3.6 kJ mol^-1) by the spectrophotometric method. The differences in the energy levels between the two geometries of the cobalt(II) complexes in the spherically symmetric field (ΔE_spher) were estimated from the values of ΔH° by offsetting the ligand field stabilization energies. It was indicated that the value of ΔE_spher is the decisive factor in determining the value of ΔH° and is largely dependent on the electronic repulsion between the d-electrons and the donor atoms and the interelectronic repulsion in the d orbitals. The comparison between activation enthalpies (ΔH^‡) for the solvent exchange reactions of octahedral cobalt(II) ions in pa and meea revealed that the unexpectedly large rate constant and small ΔH^‡ in pa are attributed to the strong electronic repulsion in the ground state and removal of the electronic repulsion in the dissociative transition state, which can give the small ΔE_spher between the ground and transition states. Differences in the solvent exchange rates and the ΔH^‡ values of the octahedral metal(II) ions in some other solvents are discussed in connection with the electronic repulsive factors.