EARLY STAGES OF THE HYDROLYSIS OF CHROMIUM(III) IN AQUEOUS-SOLUTION .9. KINETICS OF WATER EXCHANGE ON THE HYDROLYTIC DIMER

Oxygen-18-labeling techniques have been used to study the kinetics of water exchange on the Cr(III) hydrolytic dimer at I = 1.0 M, [H+] in the range 0.300-0.1 M, and temperatures between 287.2 and 305.2 K. Three distinct pathways for exchnage of oxygen-18 from the labeled dimer into the bulk solvent were observed. Two of these rate constants (k(fast) and k(slow)) correspond to the exchange of coordinated water molecules occupying positions trans and cis to the bridging OH groups. They were found to show a linear dependence on 1/[H+], k(i) = k(i)(0) + k(i)(OH)/[H+], consistent with the involvement of both fully protonated and monodeprotonated dimer. The following parameters were found: for exchange on [(H2O)(4)Cr(mu-OH)(2)Cr(H2O)(4)](4+) at 298 K, k(fast)(0) = 3.6 x 10(-4) s(-1) (Delta H* = 81 +/- 8 kJ mol(-1) and Delta S* = -40 +/- 26 J K-1 mol(-1)) and k(slow)(0) = 6.6 x 10(-5) s(-1) (Delta H* = 97 +/- 10 kJ mol(-1) and Delta S* = 0 +/- 32 J K-1 mol(-1)); for exchange on [(H2O)(4)Cr(mu-OH)(2)Cr(OH)(H2O)(3)](3+), k(fast)(OH) = 2.6 x 10(-6) s(-1) (Delta H* = 157 +/- 2 kJ mol(-1) and Delta S* = 174 +/- 8 J K-1 mol(-1)) and k(slow)(OH) = 1 x 10(-6) s(-1) (Delta H* = 114 +/- 4 kJ mol(-1) and Delta S* = 23 +/- 14 J K-1 mol(-1)). Comparison of these data with those of previous studies suggests that there is a correlation between exchange rates and OH/Cr which holds for both bridging and terminal hydroxide groups. Deprotonation has been found to cause significant labilization of all the aquo groups on the dimer regardless of whether they are cis or trans to the hydroxide bridges. Mechanistic implications of the activation parameters are discussed. A third exchange process was identified and, from the correspondence of rate and activation parameters are discussed. A third exchange process was identified and, from the correspondence of rate and activation parameters with data on the interconversion between singly and doubly bridged dimers, attributed to the release of label from the bridging groups through bridge cleavage and re-formation reactions.