Thermochemically Consistent Free Energies of Hydration for Di- and Trivalent Metal Ions

This paper uses the relationship between the standard half reduction potential, the third ionization potential, and the free energies of hydration (Delta G(hyd)) of M2+ and M3+ ions to calculate new values of Delta G(hyd) for M2+ and M3+ ions. The numbers are "thermochemically consistent"; i.e., all numbers agree with the applied thermochemical cycle. This enables the tabulation of many Delta G(hyd) derived mainly from the data compiled by Marcus, but consistent with Delta G(hyd)(H+) = 1100 kJ/mol and SHE = 4.44 V. The accuracy of the new values of Delta G(hyd)(M3+) is by definition similar to the accuracy of the experimental hydration energy of the Delta G(hyd)(M2+) used for calculation, and vice versa, i.e. the new data have the same accuracy or higher than previously reported. As a result, the literature values for Cr3+ and Au3+, and Pd2+ are substantially revised. The approach also allows the calculation of new Delta G(hyd) for metal ions such as Mn3+, Ti2+, Ag3+, Ni3+, Cu3+, and Au2+ and the theoretically interesting but experimentally inaccessible +2 ions of lanthanides. The new numbers enable a discussion of the previously unreported trend in Delta G(hyd)(M3+) for the 3d metal ions, which relates to the ligand field stabilization energies and effective nuclear charge as for the M2+ ions. The new tabulated values should be accurate with the applied assumptions to within 10 kJ/mol and may be of value for other thermochemical calculations, for interpretation of the aqueous trend chemistry of the metal ions, and as benchmarks for theoretical chemistry.