| Abstract
| - Chromium(V) oxalato complexes have been ofconsiderable interest as intermediates in the Cr(VI) oxidationofoxalic acid, which is used as a prototype in mechanistic studies ofCr(VI) oxidations of organic substrates. Thesecomplexes have been characterized by X-band EPR spectroscopy from thereactions of Cr(VI) (CrO3) or Cr(V)(Na[CrVO(ehba)2]; ehba =2-ethyl-2-hydroxybutanoate(2−)) with oxalic acid(oxH2) in acidic aqueous solutions(pH = 0−1.5; I = 1 M(HClO4/NaClO4); 21 °C). Structures andformation mechanisms of these complexes havebeen deduced from the dependences of the relative intensities of thecorresponding EPR signals (obtained bydigital simulations of the EPR spectra) on the reaction conditions.A range of Cr(V) complexes, including five-coordinate species,[CrV(O)2(OH2)(ehba)]-,[CrVO(ehba)(ox)]-, and[CrVO(ox)2]-, andsix-coordinate species,[CrVO(OH2)(ox)2]-,[CrVO(OH)(ox)2]2-,[CrVO(oxH)(ox)2]2-,and possibly[CrV(O)2(OH2)2(ox)]-,as well as mixed-valence Cr(III)−Cr(V) dimers and trimers andCr(VI)−Cr(V) species, such as[CrVCrVI(O)5(OH2)(ox)2]3-,havebeen characterized. Apart from the first detailed characterizationof these species, this is the first spectral evidencefor the existence of mixed-valence Cr(V)−Cr(VI) complexes.On the basis of quantitative analysis of EPR spectrafor the Cr(VI)−oxH2 system, Cr(VI) has beenshown to exist mainly in the form of the monooxalatocomplex,[CrVI(O)2(OH)(ox)]-in the presence of excess oxH2. The data on thestructures and reactivities of Cr(V) andCr(VI) oxalato complexes have been used to propose a new mechanismfor the Cr(VI) oxidation of oxH2 inacidic aqueous solutions.
- Structures and formation mechanisms ofCr(V) and mixed-valence (Cr(III)−Cr(V);Cr(V)−Cr(VI)) oxalato complexes have been assigned fromquantitative studies of EPR spectra for theCr(V)−ehbaH2−oxH2 andCr(VI)−oxH2 systems (ehbaH2 =2-ethyl-2-hydroxybutanoic acid; oxH2 = oxalic acid).The data have been used to propose a mechanism of Cr(VI)oxidation of oxH2 in acidic aqueoussolutions.
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