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-h ydroxybutanoic acid and its anion, chelated complexes of both Cr(IV) a nd Cr(V), both of them stabilized in the medium used, are formed. It a ppears that Cr(V) is not generated directly from the Cr(VI)-formate re action 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 presenc e of [Cl(NH3)(5)Co](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 o f the reacting system proceeds through Cr(IV) and bypasses the more us ual state Cr(III). Initial rates for formation of Cr(IV) lead to a rat e law pointing to a transition state containing the two redox partners , two ligating carboxyl groups, and two units of H+. Substitution of D CO2- for HCO2- retards formation of Cr(IV) by a factor of 3.3, whereas the solvent isotope affect, rate(D2O)/rate(H2O), favors the deuterate d system by a factor of 1.4. Our observations are in accord with a seq uence initiated by the ligation of HCrO4- to a chelate derived from th e 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.