REDOX CHEMISTRY OF CHROMIUM(IV) COMPLEXES

A short introduction summarizes scattered preparations of chromium(V) and chromium(IV) derivatives before 1975 and places these unusual states in perspective with respect to the more usual oxidation states of this element. Preparations of peroxo complexes of Cr(IV), known since the turn of the century, are described and their redox reactions, as investigated since 1988, are treated. Systems in which Cr(IV) has been detected in the course of reductions of water-soluble Cr(V) complexes are enumerated and the role of Cr(IV) in some autocatalytic systems and ''clock reactions'' is considered. The stabilization of Cr(IV) via chelation with the anions of a-hydroxy acids is described and the reactions of this type of chelate are discussed. The recent preparations of aqua-substituted Cr(IV) are mentioned and the role of the 2e(-) change Cr(IV)-->Cr(II) in the oxidations of organic substrates by Cr(VI) is indicated. Finally, experimental values for the standard potential and electron self-exchange rate for the Cr(III/IV) couple are compared with the corresponding parameters for Cr(IV/V) in a similar medium and structural implications are drawn from these rates.