Kinetics of anation of Cr(III) hydrolytic oligomers: reaction of dimer with sulfate

The reaction between sulfate and the Cr(III) hydrolytic dimer, [(H₂O)₄Cr(μ-OH)₂Cr(OH₂)₄]⁴⁺, has been followed in the pH range 3.5-4.4, [sulfate]=0.1-0.3 M, T=25 °C and I=1.0 M and shown to involve two clearly separable processes: monodentate attachment of sulfate (anation) to give [(H₂O)₄Cr(μ-OH)₂Cr(SO₄)(OH₂)₃]²⁺ followed by ring closure to give a sulfato bridged dimer, [(H₂O)₃Cr(μ-OH)₂(μ-SO₄)Cr(OH₂)₃]²⁺. Equilibrium measurements have shown that the first step is irreversible under the conditions of the kinetic measurements. Although definite conclusions about the reversibility of bridge formation could not be made this process is also likely to be irreversible. The observed rate constant for the anation process was found to be independent of [sulfate] suggesting that extensive ion-pairing occurs 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-pairs (charge 2+) and the monodentate sulfato 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_a1 for ions of this charge are usually greater than 5. From the kinetic data, rate accelerations of at least 30-fold have been estimated to accompany deprotonation of the reactants. They are of similar magnitude to those reported previously for other reactions involving the Cr(III) hydrolytic dimer, viz. dimerization of dimer to give tetramer and intramolecular 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.