The kinetics of the Cr(II) reduction of the pentaamminecobalt(III) complexes of m- and p-acetylbenzonitrile have been studied. In both cases, the rate law for reduction is -d ln [complex]/dt = kobsd[Cr(II)]. The kinetic parameters for the reaction are k(25°) = 0.28 ± 0.035 and (6 ± 2) × 103 M-1 sec-1, ΔH‡ = 3.2 ± 0.7 and -2.0 ± 1.5 kcal mol-1, and ΔS‡ = -50 ± 2 and -48 ± 5 eu for the meta and para complexes, respectively. An inner-sphere mechanism for the reduction of the para complex is indicated by the observation of a Cr(III) intermediate which rapidly aquates to give Cr(OH2)63+. The rate law for the aquation of the intermediate is -d ln [intermediate]/dt = (k′ + k″[H+]-1)[Cr(II)] + k‴[H+]-1. At 25° the values for the rate constants are 420 ± 150 M-1 sec-1, 6.8 ± 6.6 sec-1, and 0.91 ± 0.13 M sec-1 for k′, k″, and k‴, respectively. The rate differences for the reduction process are explained by considering the conjugation of the carbonyl group with the aromatic ring and the various mechanisms available for transfer of the electron from reductant to oxidant.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry