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

Abstract

The bis(chelated) complex of CrV(0) derived from the dianion (L2 −) of 2-ethyl-2-hydroxybutanoic acid is readily reduced to a bis(chelate of CrIII, featuring the monoanion (LH−) [Cr V(0)(L2−)2]−+4H++H2O+2e−→[CrIII(OH2)2(LH− 2]+ of this acid. Potentials estimated by Ghosh in 1993 for this 2e− change, E0 (pH 0) 1.32 V, Eeff (pH 3.3) 0.93 V, are in accord with the nearly irreversible reductions of the Cr(V) species (in 1∶1 ligand buffer) by Fe2+, V02+, IrCl6 3 − and I−, whereas lower values reported by Bose in 1996, E0 (pH 0) 0.84 V, Eeff (pH 3.3) 0.45 V, are potentiometrically inconsistent with these conversions. A similar discrepancy is noted for potentials for Cr(V,IV) estimated in 1996, E0 (pH 0) 0.84 V, Eeff (pH 3.3) 0.46 V, which, wholly contrary to observation, predict that the reductions of excess Cr(V) to CR(IV) by Fe2+, V02+, and I− are thermodynamically disfavored.