Study on an Estimation of the Contributed Energy to the Transfer Free Energies of Complex Ions. Part I. [Fe(CN)2n(phen)(3-n)](2-2n)+ from Water to a Water-ethanol Mixed Solvent

Abstract

Abstract The solubilities of [Fe(phen)3](ClO4)2·H2O, [Fe(CN)2(phen)2]·0.5H2O, [Fe(phen)3][Fe(CN)4phen]·8H2O, and [Fe(phen)3]2[Fe(CN)6]·3.5H2O in ethanol-water mixed solvents were measured at 25.0 ± 0.1 °C, and the transfer free energy of each complex ion from water to water-ethanol mixed solvent was calculated. From the obtained data, the transfer free energies were split into contributions from 1,10-phenanthroline-solvent, cyano ligand-solvent interaction energies, and electrostatic energies. The ratio of the contributed energy from 1,10-phenanthroline vs. transfer free energy of free (non-coordinated) 1,10-phenanthroline is almost constant at about 0.6—0.7 over the whole mole-fraction range. There is thus a possibility to estimate the contributed energy from the transfer free energy of the free ligand in the future. On the other hand, the contributed energy from the cyano ligand can not be obtained by multiplying a certain coefficient to the transfer free energy of the free cyanide ion. The contributed energy from electrostatic energy changes in a somewhat different manner, as expected by the Born equation. However, the estimated transfer free energy as the sum of each contributed energy obtained by a least-squares method is in fair agreement with the observed transfer free energy of each complex ion. It has been confirmed that the proposed conception of the contributed energy to the transfer free energy is a useful means to predict the unknown transfer free energy of complex ions.