KINETICS OF ANATION OF CR(III) HYDROLYTIC OLIGOMERS - REACTION OF DIMER WITH SULFATE

The reaction between sulfate and the Cr(III) hydrolytic dimer, [(H2O)( 4)Cr(mu-OH)(2)Cr(OH2)(4)](4+), has been followed in the pH range 3.5-4 .4, [sulfate]=0.1-0.3 M, T=25 degrees C and I=1.0 M and shown to invol ve two clearly separable processes: monodentate attachment of sulfate (anation) to give [(H2O)(4)Cr(mu-OH)(2)(mu-SO4)Cr(OH2)(3)](2+) followe d by ring closure to give a sulfate bridged dimer, [(H2O)(3)Cr(mu-OH)( 2)(mu-SO4)Cr(OH2)(3)](2+). Equilibrium measurements have shown that th e first step is irreversible under the conditions of the kinetic measu rements. Although definite conclusions about the reversibility of brid ge formation could not be made this process is also likely to be irrev ersible. The observed rate constant for the anation process was found to be independent of [sulfate] suggesting that extensive ion-pairing o ccurs between the oppositely charged reactants. The rate constants for both anation and bridge formation were found to be linearly dependent on 1/[H+]. This is attributed to deprotonation of dimer-sulfate ion-p airs (charge 2+) and the monodentate sulfate complex (charge 2+). The linear dependence also suggests that the concentration of deprotonated species is low in the pH range 3.5-4.4. This is reasonable since pK(a 1) for ions of this charge are usually greater than 5. From the kineti c data, rate accelerations of at least 30-fold have been estimated to accompany deprotonation of the reactants. They are of similar magnitud e to those reported previously for other reactions involving the Cr(II I) hydrolytic dimer, viz. dimerization of dimer to give tetramer and i ntramolecular interconversion between singly and doubly bridged dimer forms. It is concluded that deprotonated sulfato-dimer species play an important part in the tanning process since their greater reactivity in substitution processes will aid in the cross-linking of collagen.