ELECTRON-TRANSFER .125. INTERMEDIATE OXIDATION-STATES IN THE REDUCTION OF CHROMIUM(VI) WITH HYPOPHOSPHITE

When HCrO4- is reduced by hypophosphite in solutions buffered by 2-eth yl-2-hydroxybutanoic acid and its anion, chelated complexes of both Cr (V) and Cr(IV), both of them stabilized in the medium used, are formed in parallel reactions. When these reactions are allowed to proceed to completion in the presence of Cl(NH3Co2+, a scavenger for Cr(II), 91- 92% of the Cr(VI) taken is found to be converted to Cr(II), indicating that very nearly all of the reacting system proceeds through Cr(IV) a nd bypasses the more usual state Cr(III). Measured initial rates for f ormation of the strongly absorbing state Cr(IV) yield a two-term rate law pointing to paths at two different protonation levels, both involv ing a transition state containing the two redox partners and two ligat ing carboxyl ,stoups. The formation of Cr(V) proceeds 5.3 times as rap idly as the generation of Cr(IV), a rate ratio essentially independent of reagent and buffer concentrations. Substitution of D2PO2- for H2PO 2- retards formation of Cr(IV) 4-fold and generation of Cr(V) by a fac tor of 2.2. The solvent isotope effect, (rate)(D2O)/(rate)(H2O), favor s the deuterated system, the ratio being 2.2 for formation of Cr(TV) a nd 1.7 for generation of Cr(V). Our observations favor a sequence init iated by the ligation of HCrO4- to a bis chelate of Cr(VI) derived fro m the buffering carboxylate anion. Conversions of Cr(VI) to Cr(IV) and of Cr(IV) to Cr(II) entail hydride shifts from P(I) to the Cr(=O) fun ction, whereas the formation of Cr(V) and its reduction of the latter to Cr(IV) may involve preliminary coordination of H2PO2- to the chromi um center, followed by P-H to O-H tautomerization within the binuclear complex and then single-electron transfer from phosphorus to the chro mium center.