Electron transfer. 107. Chromium(VI)-Uranium(IV) reaction. Parallel one- and two-electron paths

Reduction of HCrO4- with U(IV) in solutions buffered by 2-ethyl-2-hydroxybutanoic acid (HLig) and its salt (Lig-) is rapid and leads to a mixture of bis chelates of Cr(IV), Cr(V), and Cr(III), each derived from the buffering anion. The ratio of products approaches 3:1:1 and is nearly independent of [Lig-]. Kinetic patterns suggest that the complexes UIV (Lig-) and UIV(Lig-)2 are active reductants but that UIV(Lig-)3 is inactive. With U(IV) in excess, Cr(IV) is slowly reduced to Cr(III), a reaction which must involve intervention of U(V). The U(IV)-Cr(IV) reaction appears to proceed through a precursor complex (Kassn = 5 x 103 M-1), the pool of which is depleted both by protonation and extraligation. With Cr(VI) in excess, Cr(IV) is slowly lost by comproportionation (CrIV + CrVI → 2CrV), a transformation which has been observed also in Cr(VI)-As(III) and Cr(VI)-Mo2V systems. Cr(III), formed quickly in the initial stage of conversion in quantity equivalent to Cr(V), cannot arise from the Cr(V)-U(IV) or the Cr(IV)-U(IV) reaction, both of which are too slow, but is instead produced by reduction of Cr(IV) with U(V). ESR studies show that rapid reductions of excess HCrO4- with Mo2O42+, HSO3-, or Sn(II) in the authors buffer system likewise yield, along with major quantities of Cr(IV), minor amounts of Cr(V), thus pointing to competitive 1e- and 2e- transactions with these reductants. Reduction by As(III) under the same conditions, however, yields only Cr(IV)