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 t he range 0.300-0.1 M, and temperatures between 287.2 and 305.2 K. Thre e distinct pathways for exchnage of oxygen-18 from the labeled dimer i nto the bulk solvent were observed. Two of these rate constants (k(fas t) and k(slow)) correspond to the exchange of coordinated water molecu les 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 a nd monodeprotonated dimer. The following parameters were found: for ex change 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 excha nge on [(H2O)(4)Cr(mu-OH)(2)Cr(OH)(H2O)(3)](3+), k(fast)(OH) = 2.6 x 1 0(-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 th ese data with those of previous studies suggests that there is a corre lation between exchange rates and OH/Cr which holds for both bridging and terminal hydroxide groups. Deprotonation has been found to cause s ignificant labilization of all the aquo groups on the dimer regardless of whether they are cis or trans to the hydroxide bridges. Mechanisti c implications of the activation parameters are discussed. A third exc hange process was identified and, from the correspondence of rate and activation parameters are discussed. A third exchange process was iden tified and, from the correspondence of rate and activation parameters with data on the interconversion between singly and doubly bridged dim ers, attributed to the release of label from the bridging groups throu gh bridge cleavage and re-formation reactions.