Electron transfer. 138. Potentials involving chelated chromium(V)

The bis(chelated) complex of Cr-V(0) derived from the dianion (L2-) of 2-et hyl-2-hydroxybutanoic acid is readily reduced to a bis(chelate of Cr-III, f eaturing the monoanion (LH-) [Cr-V(0)(L2-)(2)](-) + 4H(+) + H2O + 2e(-) --> [Cr-III(OH2)(2)(LH-)(2)](+) of this acid. Potentials estimated by Ghosh in 1993 for this 2e(-) change, E-0 (pH 0) 1.32 V, E-eff (pH 3.3) 0.93 V, are in accord with the nearly irreversible reductions of the Cr(V) species tin 1:1 ligand buffer) by Fe2+, V0(2+), IrCl63- and I- whereas lower values rep orted by Bose in 1996, E-0 (pH 0) 0.84 V, E-eff (pH 3.3) 0.45 V, are potent iometrically inconsistent with these conversions. A similar discrepancy is noted for potentials for Cr(V,IV) estimated in 1996, E-0 (pH 0) 0.84 V, E-e ff (pH 3.3) 0.46 V, which, wholly contrary to observation, predict that the reductions of excess Cr(V) to CR(IV) by Fe2+, V0(2+), and I- are thermodyn amically disfavored.