Hydrolysis of the Tripositive Ions of the First Transition Series : a Discussion of the Heats and Entropies for the Dissociation of the First Proton

Abstract

THERMODYNAMIC data are now available for the reaction for all members of the first transition series which form a reasonably stable hexa-aquo ion M³⁺ in solution except titanous. These are collected in Table 1 for perchlorate media, where possible at ionic strength µ = 1.0: the equilibrium constant K = [MOH²⁺] [${\mathrm{H}}_3^{\text{+}}\mathrm{O}$]/[M³⁺] at 25° C is also included, the value for titanous at µ = 1.0 being interpolated from the data of Pecsok and Fletcher⁶. As the first hydrolysis product MOH²⁺ must be an inner sphere complex⁷, changes in ΔH for reaction (1) along the first transition series might be expected to arise from changes in the difference between the crystal field stabilizations produced by H2O and OH . The crystal field splitting is greater for H₂O than OH⁻ and in general should result in stabilization of the hexa-aquo ion: this stabilization is absent for the $t_{2_g}^3{e}_g^2$ arrangement, and therefore K for the tripositive ions in the first transition series should have a maximum at ferric ion. However, Table 1 shows clearly that this is not the case, indicating that the influence of differences in crystal field stabilization on reaction (1) must be small. In fact, the spectral data⁸ for the rhodium(III) complexes Rh(NH₃)₅H₂O³⁺ and Rh(NH₃)₅OH²⁺, where the wave-lengths of maximum absorption are 31,600 cm⁻¹ and 31,200 cm⁻¹, respectively, suggest that the difference in stabilization produced by H₂O and OH⁻ will be small enough to neglect when considering changing influences on reaction (1) along a transition series.