Fast ligand substitution at a chromium(III) hydroperoxo complex.

The hydroperoxo group in (H(2)O)(5)CrOOH(2+) is replaced with selected anions (acetate, nitrate, and sulfate) at rates that are unprecedented for substitution at a pentaaquachromium(III) complex. The chemistry is noteworthy in that it interchanges low-concentration, weak ligands between a Cr(III) coordination sphere and solution in the presence of a large excess of water. We attribute this chemistry and the fast kinetics to the hydroperoxo cis effect. Each of the entering anions follows a different rate law in support of H(+)-assisted dissociation of the hydroperoxo group from a mixed-ligand intermediate. The dependence on [H(+)] is first-order for nitrate, zero-order for acetic acid, and changes from first- to zero-order for sulfate in the acidity range 0.015 M < [H(+)] < 0.20 M. At 1.0 M H(+), the second-order rate constants are 3.0 (nitrate), 2.49 (sulfate), and 0.90 (acetic acid) M(-1) s(-1). The similarity in rate constants supports a mechanism featuring rate-limiting substitution at chromium. The equilibrium constant for the reaction (H(2)O)(5)CrOOH(2+) + NO(3)(-) + H(+) <==> (H(2)O)(5)CrONO(2)(2+) + H(2)O(2), K = 62 M(-1), was obtained from the kinetics of forward and reverse reactions.