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 stud ied in the pH range 3.2-5.1 using a pH-stat facility which monitored t he volume of base consumed during reaction at constant pH. The reactio n, found to be irreversible, was followed for up to 5% conversion of r eactants to ensure that trimer was a major product. The pH dependence of k(obs), after correction for dimerization of monomer and dimer to d imer and tetramer, respectively, has been fitted to an expression whic h 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)]; e ither Cr(OH)(2)(+) + Cr-2(mu-OH)(2)(OH)(2)(2+) --> ((k(12) less than o r 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 deprotonat ion of both reactants (k(22)) gave a 10,000-fold increase in rate comp ared with the monodeprotonated reactants (k(11)). These increases in r eactivity on deprotonation are consistent with a common trend observed in other hydrolytic processes of chromium(III).