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 mon omer 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 format ion reactions were irreversible and could be followed for up to 5-10% conversion of reactants without interference from competing polymeriza tion processes. The pH dependence of k(obs) for hexamer and tetramer f ormation was successfully fitted to expressions which accommodated fou r 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+) --> hexame r 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 e qual 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 for mation could involve either of two processes (k(12) or k(21)) which ca nnot be distinguished from the kinetic data. Of all the oligomer forma tion reactions studied thus far, the dimerization of trimer is the fir st system where a triply deprotonated reactant has been involved in th e polymerization process. The observed increases in reactivity result mainly from labilization of the primary coordination sphere of chromiu m(III) on deprotonation of the reactants and is consistent with a comm on trend observed for other hydrolytic processes for chromium(III). Co mparison of all the data on the polymerization of chromium(III) indica tes that reactions between dissimilar reactants (dimer+monomer and tri mer+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.