Chromium from inner sphere oxidation

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

The kinetics of chromium(l 11 )-catalyscd oxidation of fonnic acid by Ce(TV) in aqueous H2SO4 can be rationalized in terms of initial formation of an outer-sphere complex involving oxidant, catalyst, and substrate (S), Ce(TV)(S)Cr(III), followed by an inner-sphere complex Ce(III)(S)Cr(IV). It is proposed that electron transfer occurs within this complex from substrate to Cr(TV) (with elimination of H+) followed by fast reaction to give CO2 (again with elimination of H+).54 In contrast, there was no kinetic evidence for the accumulation of a corresponding inner-sphere intermediate in the osmium(VIII)-catalysed Ce(TV) oxidation of DMSO to dimethyl sulfone here, the observed rate law was rationalized in terms of rate-determining bimolecular electron transfer from DMSO to Os(VHI) in an outer-sphere step.55 The kinetics of oxidation of 2-hydroxy-l-naphthalidene anil by cerium(IV) in aqueous sulfuric acid have been... 

The five-coordinate cobalt(II) species presumably immediately picks up a water molecule to fill its sixth coordination position and then hydrolyzes rapidly to [Co(H20)J . Formally, such an inner sphere reaction consists of the transfer of a chlorine atom from cobalt to chromium, decreasing the oxidation stale of the former but increasing that of the latter. In addition to the self-consistency of the above model (inert and labile species) and the observed formation of a chlorochromium complex, further evidence for this mechanism has been obtained by running the reaction in the presence of free radioisotopes of chloride ion in the solution. Very little of this labeled chloride is ever found in the product, indicating that the chloride transfer has indeed been through the bridge rather than indirectly through free chloride. 

These studies differ in that, whereas Wells and co-workers have spectrally characterised an intermediate and suggested from the data that the substitution rate for Mn" is 5 x 10 1 mol s" (c/. the recent study by Diebler on the formation of fluoride species), since it should lie between that for the chromium(ii) species and the oxidation rate for the Mn" -hydroquinone complex, Davies and Kustin postulate in the acid range 0-6—3-60 mol 1 an inner-sphere mechanism with hydrogen atom transfer as the predominant mode of reaction for MnOH +, with rates not too different from those derived from studies on other systems (Table 1). The reaction is first-order in each reactant and the observed second-order rate constant is invariant with [Mn" ]o, [H2Q], and [Mn"]. A mechanism consistent with intermediate formation may be written... 

As noted in Section 6.5, the MW and MW distribution obtained from Cr/ silica are sensitive to small changes in the catalyst s calcination history. This response indicates that oxide atoms of the support occupy the inner coordination sphere of the chromium. The active sites are influenced by... 

---