KINETIC AND THERMODYNAMIC STUDIES OF INTRAMOLECULAR REARRANGEMENT ANDCLEAVAGE OF THE HETEROBINUCLEAR AQUA ION, [(H2O)(4)RH(MU-OH)(2)CR(OH2)(4)](4+)

The doubly bridged (d.b.) heterobinuclear aqua ion [(H2O)(4)Rh(mu-OH)( 2)Cr(OH2)(4)](4+) was converted into a singly bridged (s.b.) form [(H2 O)(5)Rh(mu-OH)Cr(OH2)(5)](5+) in strongly acidic solution. Pure soluti ons of the latter were obtained using low-temperature ion-exchange chr omatography, 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 measur able concentrations, allowing equilibrium measurements to be made. The variation in the relative concentrations of the two forms with [H+] h as been interpreted in terms of two equilibria: deprotonation/protonat ion 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 h igh charge and hydrogen-bond stabilisation of s.b.;H. The pH dependenc e 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 wi thin protonated d.b. and d.b. Only small enhancements in ring-closure rates were observed on deprotonation of the s.b. form when compared wi th the related chromium(III) systems. Acid cleavage of [(H2O)(4)Rh(mu- OH)(2)Cr(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. int o the s.b. form. The similarity in the rates and activation parameters , for both the interconversion and cleavage reactions, to those report ed previously for the corresponding chromium(III) dimer indicates that the reactions of [(H2O)(4)Rh(mu-OH)(2)Cr(OH2)(4)](4+) are Cr-III cent red substitution processes.