EARLY STAGES OF THE HYDROLYSIS OF CHROMIUM(III) IN AQUEOUS-SOLUTION .10. KINETICS OF FORMATION OF TRIMER FROM MONOMER AND DIMER

Conversion of chromium(III) monomer and dimer to trimer have been studied in the pH range 3.2-5.1 using a pH-stat facility which monitored the volume of base consumed during reaction at constant pH. The reaction, found to be irreversible, was followed for up to 5% conversion of reactants to ensure that trimer was a major product. The pH dependence of k(obs), after correction for dimerization of monomer and dimer to dimer and tetramer, respectively, has been fitted to an expression which accommodates the three reaction pathways: Cr(OH)(2+) + Cr-2(mu-OH)(2)(OH)(3+) --> trimer [k(11) = (3.13 +/- 0.30) x 10(-4) M(-1) s(-1)]; either Cr(OH)(2)(+) + Cr-2(mu-OH)(2)(OH)(2)(2+) --> ((k(12) less than or equal to 1.16 x 10(-1) M(-1) s(-1)) or Cr(OH)(2)(+) + Cr-2(mu-OH)(2)(OH)(3+) --> trimer (k(21) less than or equal to 2.09 x 10(-1) M(-1) s(-1)); and Cr(OH)(2)(+) + Cr-2(mu-OH)(2)(OH)(2)(2+) --> trimer [k(22) = (3.12 +/- 0.67) M(-1) s(-1)]. Reaction between one monodeprotonated and one doubly deprotonated reactant is 400 times faster than reaction between the two monodeprotonated reactants (k(11)). Double deprotonation of both reactants (k(22)) gave a 10,000-fold increase in rate compared with the monodeprotonated reactants (k(11)). These increases in reactivity on deprotonation are consistent with a common trend observed in other hydrolytic processes of chromium(III).