Kinetics and mechanisms of dissociation of metal chelates. Part 3. Effects of ions on the acid hydrolysis of tris(1,10-phenanthroline)iron(II) perchlorate

Abstract

The rate of acid hydrolysis of the title complex cation, [Fe(phen)₃]²⁺, has been investigated in the presence of varying concentrations of HCl, LiCl, NaCl, KCl, [NH₄]Cl, NaBr, KBr, [NH₄]Br, MgCl₂, CaCl₂, SrCl₂, BaCl₂, and MgBr₂. In each system, the pseudo-first-order rate constant, k_H2O, decreases linearly with increasing molality, m, of the added electrolyte. The degree of retardation is measured by the retardation constant, k_r=ε/k₀, where ε is the slope of the mean straight line obtained by least square from plots of k_H2Ovs. m of the added salt, and k₀ is k_H2O for m= 0. The constant k_r is linearly related to enthalpies and entropies of hydration, crystallographic radii of the added cations, reorientation times of water molecules around the ions, etc. It was concluded, as expected, that the degree of retardation of the rate of hydrolysis is related to the degree of formation of the ‘structure of liquid water’ induced by the added ions. Ion-pair formation and nucleophilic action as well as charge–radius effects are suggested as contributing to the effects of anions.