ELECTRON-TRANSFER .103. REACTIONS OF AQUEOUS CHROMIUM(IV)

Reduction of HCrO4−with the bridged dimer of Mov(Mo2O42+) in aqueous solutions buffered by 2-ethyl-2-hydroxybutanoic acid (HLig) and its salt (Lig−) yields a pink Cr(IV) complex (ϵ510max= 1.3 × 103M−1cm−1when [Lig−] = 0.05 M). This species is reduced to a bichelate of CrIIIwith excess (Mov)2but oxidized to a bichelate of Crvwith excess Cr2072−, indicating that it too is bischelated. The formation of CrIvfrom CrvI and (Mov)2proceeds through two routes, with the activated complex for the second featuring one more H+and one more carboxylate group than that for the first. The comproportionation reaction, CrIV+ CrVI→ 2Crv, appears to entail a CrIvCrvIcomplex (Kassn= 40 ± 7 M−1), which reacts via both an acid-independent and a [H+]-proportional path. Data for the reaction of CrIvwith (Mov)2are in accord with the sequence (6)−(9) in which the key step is the reversible, unimolecular conversion to the principal (Mov)2species to an activated form (k = 5 × 102s−1at 25 °C) that reacts rapidly with CrIv. To form a CrIvcomplex of this type stable enough to be examined in conventional mixing experiments, it is necessary that the carboxylate ligand be attached to the CrIVcenter prior to reduction and that the usual le−reductants, which rapidly reduce CrIV, be avoided. © 1990, American Chemical Society. All rights reserved.