KINETICS OF WATER EXCHANGE ON THE DIHYDROXO-BRIDGED RHODIUM(III) HYDROLYTIC DIMER

Conventional O-18 isotopic labeling techniques have been used to measu re the water exchange rates on the Rh(III) hydrolytic dimer [(H2O)(4)R h(mu-OH)(2)Rh(H2O)(4)](4+) at I = 1.0 M for 0.08 < [H+] < 0.8 M and te mperatures between 308.1 and 323.1 K. Two distinct pathways of water e xchange into the bulk solvent were observed (k(fast) and k(slow)) whic h are proposed to correspond to exchange of coordinated water at posit ions cis and trans to bridging hydroxide groups. This proposal is supp orted by O-17 NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of ex change matched those from the O-18 labeling data. No evidence was foun d for the exchange of label in the bridging OH groups in either experi ment. This contrasts with findings for the Cr(III) dimer. The dependen ce of both k(fast) and k(slow) on [H+] satisfied the expression k(obs) = (k(o)[H+](tot) +k(oH)K(al))/([H+](tot) + K-al) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. T he following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer: k(fast) = 1.26 x 10(-6) s(-1) (Delta H double dagger = 119 +/- 4 kJ mol(-1) and Delta S = 41 +/- 12 J K-1 mol(-1)) and k(slow) = 4.86 x 10(-7) s(-1) (Delta H double dagger = 64 +/- 9 kJ mol(-1) and Delta S double dagger = -150 +/- 30 J K-1 mol(-1 )). (ii) For monodeprotonated dimer: k(fast) = 3.44 x 10(-6) s(-1) (De lta H double dagger = 146 +/- 4 kJ mol(-1) and Delta S double dagger = 140 +/- 11 J K-1 mol(-1)) and k(slow) = 2.68 x 10(-6) s(-1) (Delta H double dagger = 102 +/- 3 kJ mol(-1) and Delta S double dagger = -9 +/ - 11 J K-1 mol(-1)). Deprotonation of the Rh(III) dimer was found to l abilize the primary coordination sphere of the metal ions and thus inc rease the rate of water exchange at positions cis and trans to bridgin g hydroxides but not to the same extent as for the Cr(III) dimer. Acti vation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trival ent metal ions and in particular the Cr(III) dimer.