Early stages of the hydrolysis of chromium(III) in aqueous solution .11. Kinetics of formation of hexamer from trimer and tetramer from monomer and trimer

The kinetics of conversion of chromium(III) trimer to hexamer, and monomer and trimer to tetramer have been studied in the pH range 3.4-5.2 using a pH-stat facility. Product analyses showed that oligomer formation reactions were irreversible and could be followed for up to 5-10% conversion of reactants without interference from competing polymerization processes. The pH dependence of k(obs) for hexamer and tetramer formation was successfully fitted to expressions which accommodated four and three reaction pathways, respectively: Cr-3(mu-OH)(4)(OH)(4+) + Cr-3(mu-OH)(4)(OH)(4+) --> hexamer k(11) = 8.51 (+/-0.59) x 10(-4) M(-1) s(-1) Cr-3(mu-OH)(4)(OH)(2)(3+) + Cr-3(mu-OH)(4)(OH)(4+) --> hexamer k(21) = 9.7 (+/-2.6) x 10(-3) M(-1) s(-1) Cr-3(mu-OH)(4)(OH)(2)(3+) + Cr-3(mu-OH)(4)(OH)(2)(3+) --> hexamer k(22) = 4.59 (+/-0.75) x 10(-1) M(-1) s(-1) Cr-3(mu-OH)(4)(OH)(3)(2+) + Cr-3(mu-OH)(4)(OH)(2)(3+) --> hexamer k(32) = 1.07 (+/-0.41) M(-1) s(-1) Cr(OH)(2+) + Cr-3(mu-OH)(4)(OH)(4+) --> tetramer k(11) = 1.89 (+/-0.17) x 10(-3) M(-1) s(-1) Cr(OH)(2+) + Cr-3(mu-OH)(4)(OH)(2)(3+) --> tetramer k(12) less than or equal to 2.96 (+/-0.29) x 10(-1) M(-1) s(-1) Cr(OH)(2+) + Cr-3(mu-OH)(4)(OH)(4+) --> tetramer k(21) less than or equal to 8.20 (+/-0.62) x 10(-1) M(-1) s(-1) Cr(OH)(2+) + Cr-3(mu-OH)(4)(OH)(2)(3+) --> tetramer k(22) = 4.80 (+/-0.56) M(-1) s(-1). The second pathway for tetramer formation could involve either of two processes (k(12) or k(21)) which cannot be distinguished from the kinetic data. Of all the oligomer formation reactions studied thus far, the dimerization of trimer is the first system where a triply deprotonated reactant has been involved in the polymerization process. The observed increases in reactivity result mainly from labilization of the primary coordination sphere of chromium(III) on deprotonation of the reactants and is consistent with a common trend observed for other hydrolytic processes for chromium(III). Comparison of all the data on the polymerization of chromium(III) indicates that reactions between dissimilar reactants (dimer+monomer and trimer+monomer) are faster than the dimerization of individual oligomers (monomer, dimer and trimer) and thus are the preferred pathways in the polymerization of Cr3+. (C) 1996 Elsevier Science Ltd.