Kinetic and thermodynamic studies of intramolecular rearrangement and cleavage of the heterobinuclear aqua ion, [(H2O)4Rh(µ-OH)2Cr(OH2)4]4+

Abstract

The doubly bridged (d.b.) heterobinuclear aqua ion [(H₂O)₄Rh(µ-OH)₂Cr(OH₂)₄]⁴⁺ was converted into a singly bridged (s.b.) form [(H₂O)₅Rh(µ-OH)Cr(OH₂)₅]⁵⁺ in strongly acidic solution. Pure solutions of the latter were obtained using low-temperature ion-exchange chromatography. Its cleavage is much slower than interconversion between the d.b. and s.b. forms. In the range pH 0–1 both forms exist in measurable concentrations, allowing equilibrium measurements to be made. The variation in the relative concentrations of the two forms with [H⁺] has been interpreted in terms of two equilibria: deprotonation/protonation of the s.b. form and equilibration between monodeprotonated (s.b. – H) and d.b. forms. The high acidity of the s.b. form is due to the high charge and hydrogen-bond stabilisation of s.b. – H. The pH dependence of k_obs for the interconversion between the s.b. and d.b. forms of the aqua ion has been interpreted in terms of five reaction pathways: ring closure within s.b., s.b. – H and s.b. – 2H, and ring opening within protonated d.b. and d.b. Only small enhancements in ring-closure rates were observed on deprotonation of the s.b. form when compared with the related chromium(III) systems. Acid cleavage of [(H₂O)₄Rh(µ-OH)₂Cr(OH₂)₄]⁴⁺ to Cr³⁺ and Rh³⁺ was found to proceed via the s.b. and s.b. – H forms, the acid dependence of the rate constant being due to equilibration between the s.b. and d.b. forms. Cleavage of the s.b. form is at least 100 times slower than conversion of the d.b. into the s.b. form. The similarity in the rates and activation parameters, for both the interconversion and cleavage reactions, to those reported previously for the corresponding chromium(III) dimer indicates that the reactions of [(H₂O)₄Rh(µ-OH)₂Cr(OH₂)₄]⁴⁺ are Cr^III centred substitution processes.