Kinetics of the reaction of chromium(VI) with tris(1,10-phenanthroline)iron(II) ions in acidic solutions. Anion and medium effects: perchlorate versus triflate.

Reinvestigation of the reaction between title reagents in aqueous acidic triflate and perchlorate media revealed an unusual difference: the reaction is strictly first-order with respect to the concentration of Fe(phen)3(2+) (phen = 1,10-phenanthroline) in the triflate medium but shows an additional, but we believe artifactual, higher-order term in the perchlorate medium. We postulate that the apparent orders with respect to [Fe(phen)3(2+)] in (H/Li)ClO4 do not indicate the actual chemical mechanism but, in whole or in part, the orders, particularly the higher-order component, reflect an interaction specific to Fe(phen)3(2+) or Fe(phen)3(3+) and ClO4(-) in solution. Data in (H/Li)O3SCF3 solutions indicate that, in the absence of added Fe(phen)3(3+), the first of the three sequential electron-transfer steps is rate controlling. Reactions started in the presence of the product Fe(phen)3(3+) occur somewhat more slowly, suggesting the first electron transfer is reversible. This finding allows the relative rate constants for Cr(V) oxidation and reduction to be evaluated, with limited precision, by two methods of analysis. The dependences on [Cr(VI)] can be resolved into contributions from the species HCrO4(-) and Cr2O7(2-), each of which in turn depends on [H+]. The reaction mechanism is discussed in light of the data obtained in the triflate medium. Further, the rate constants for certain steps can be considered in light of E0 for the Cr(VI)/(V) couple.