Kinetics of Water Exchange on the Dihydroxo-Bridged Rhodium(III) Hydrolytic Dimer

Conventional 18O isotopic labeling techniques have been used to measure the water exchange rates on the Rh(III) hydrolytic dimer [(H2O)4Rh(μ-OH)2Rh(H2O)4]4+ at I = 1.0 M for 0.08 < [H+] < 0.8 M and temperatures between 308.1 and 323.1 K. Two distinct pathways of water exchange into the bulk solvent were observed (k fast and k slow) which are proposed to correspond to exchange of coordinated water at positions cis and trans to bridging hydroxide groups. This proposal is supported by 17O NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of exchange matched those from the 18O labeling data. No evidence was found for the exchange of label in the bridging OH groups in either experiment. This contrasts with findings for the Cr(III) dimer. The dependence of both k fast and k slow on [H+] satisfied the expression k obs = (k O[H+]tot +k OH K a1)/([H+]tot + K a1) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. The following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer: k fast = 1.26 × 10-6 s-1 (ΔH * = 119 ± 4 kJ mol-1 and ΔS * = 41 ± 12 J K-1 mol-1) and k slow = 4.86 × 10-7 s-1 (ΔH * = 64 ± 9 kJ mol-1 and ΔS * = −150 ± 30 J K-1 mol-1). (ii) For monodeprotonated dimer: k fast = 3.44 × 10-6 s-1 (ΔH * = 146 ± 4 kJ mol-1 and ΔS * = 140 ± 11 J K-1 mol-1) and k slow = 2.68 × 10-6 s-1 (ΔH * = 102 ± 3 kJ mol-1 and ΔS * = −9 ± 11 J K-1 mol-1). Deprotonation of the Rh(III) dimer was found to labilize the primary coordination sphere of the metal ions and thus increase the rate of water exchange at positions cis and trans to bridging hydroxides but not to the same extent as for the Cr(III) dimer. Activation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trivalent metal ions and in particular the Cr(III) dimer.