Electron transfer. 127 Intermediate oxidation states in the reduction of chromium(VI) with formate

When HCrO₄ ^? is reduced by formate in solutions buffered by 2-ethyl-2-hydroxybutanoic acid and its anion, chelated complexes of both Cr(IV) and Cr(V), both of them stabilized in the medium used, are formed. It appears that Cr(V) is not generated directly from the Cr(VI)-formate reaction but arises instead from oxidation of Cr(IV) by Cr(VI). When the Cr(VI)-formate reaction is allowed to go to completion in the presence of [Cl(NH₃)₅Co]²⁺, a scavenger for Cr(II), 84–86% of the Cr(VI) taken is found to be converted to Cr(II), indicating that nearly all of the reacting system proceeds through Cr(IV) and bypasses the more usual state Cr(III). Initial rates for formation of Cr(IV) lead to a rate law pointing to a transition state containing the two redox partners, two ligating carboxyl groups, and two units of H⁺. Substitution of DCO₂ ^? for HCO₂ ^? retards formation of Cr(IV) by a factor of 3.3, whereas the solvent isotope affect, rate_{D 2}O/rate_{H 2}O, favors the deuterated system by a factor of 1.4. Our observations are in accord with a sequence initiated by the ligation of HCrO₄ ^? to a chelate derived from the buffering carboxylate anion. Conversions of Cr(VI) to Cr(IV), and Cr(IV) to Cr(II) appear to entail hydride shifts from formate to the Cr(=O) function.