Cobalt redox potentials

When the water ligands around a cation are replaced by other ligands which are more strongly attached, the redox potential can change dramatically, for example for the cobalt(II)-cobalt(III) system we have... 

Cobalt, tris(l,2-ethanediamine)-conformation, 1,25,197 polarography, 1,481 racemization, 1, 466 solid state, 1,466,467 reactions, 1, 27 redox potential, 1, 514 structure, 1, 67... 

Similar effects are observed in the iron complexes of Eqs. (9.13) and (9.14). The charge on the negatively charged ligands dominates the redox potential, and we observe stabilization of the iron(iii) state. The complexes are high-spin in both the oxidation states. The importance of the low-spin configuration (as in our discussion of the cobalt complexes) is seen with the complex ions [Fe(CN)6] and [Fe(CN)6] (Fq. 9.15), both of which are low-spin. 

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

Late transition metal or 3d-transition metal irons, such as cobalt, nickel, and copper, are important for catalysis, magnetism, and optics. Reduction of 3d-transition metal ions to zero-valent metals is quite difficult because of their lower redox potentials than those of noble metal ions. A production of bimetallic nanoparticles between 3d-transi-tion metal and noble metal, however, is not so difficult. In 1993, we successfully established a new preparation method of PVP-protected CuPd bimetallic nanoparticles [71-73]. In this method, bimetallic hydroxide colloid forms in the first step by adjusting the pH value with a sodium hydroxide solution before the reduction process, which is designed to overcome the problems caused by the difference in redox potentials. Then, the bimetallic species... 

Sithambaram, S., Garces, H.F. and Suib, S.L. (2009) Controlled synthesis of self-assembled metal oxide hollow spheres via tuning redox potentials versatile nanostructured cobalt and cobalt manganese oxides. Advanced Materials, 20, 1205-1209. 

Jensen was the first to report in 1983 that the color of the solution oscillated between pink and dark brown in the presence of cobalt(II) and bromide ions when the reaction was carried out in a 90/10 (w/w) acetic acid/water mixture (162). This color change was accompanied by a change in the redox potential and the oscillations were observed for over 16 h and 800 cycles. Presumably, the pink color corresponds to a low Co(III)/Co(II) ratio, the dark brownish black to a high Co(III)/Co(II) ratio or to a Co(III)Br complex in this reaction. 

SAQ 7.10 Consider the cobaltous ion cobalt redox couple. Write an expression for its electrode potential. 

On the other hand, the axial EPD molecules only slightly affect the cobalt(II)-cobalt(I)redox potentials, since the interactions of both cobalt(II) and cobalt (I) with strongly basic Hgands are about equally weak. On the other hand, the Co(II)-Co(I) redox potentid is strongly influenced by the donicity of the equatorial ligand. A linear free relationship was found between the half-wave potential E1/2 and the nucleo-philicity of the Co (I) species ... 

With CoTAA, the stationary curve obtained in 2 N H2SO4 under N2 shows an anodic and a cathodic peak, which should probably be assigned to the change in valency of cobalt (Co2+ Co3+) n). The redox potential of this process lies at about 650 mV. The oxygen, contrary to Beck s suppositions, is reduced at CoTAA electrodes far above this potential, with relatively high current densities and in a potential region in which the reactions... 

We are still further from being able to explain the anodic activity of the CoTAA complex. The cobalt phthalocyanine, which is structurally identical with CoTAA in the inner coordination sphere, is completely inactive in the catalysis of anodic reactions. It therefore looks as if the central region is not exclusively responsible for the anodic activity. On the other hand, the fact that CoTAA is inactive for the oxidation of H2 points to n orbitals of the fuel participating in the formation of the chelate-fuel complex. A redox mechanism (cf. Section 5.2) can be ruled out because anodic oxidation proceeds only in the region below the redox potential of CoTAA (i.e. at about 600—650 mV). 

The resultant hydroxyl radicals are effective in initiating many chain reactions. The number of metal ions and complexes which are capable of activating hydrogen peroxide in this manner is quite large and is determined in part by the redox potentials of the activator. Related systems in which free radicals are generated by the intervention of suitable metallic catalysts include many in which oxygen is consumed in autoxidations. Cobalt(H) compounds which act as oxygen carriers can often activate radicals in such systems by reactions of the type ... 

Its oxidation potential is such that it is oxidized to the kinetically inert 2 1 cobalt(III) complex (equation 6). The relevant redox potentials have not been measured, but support is provided by the isolation by Wittwer46 of 2 1 cobalt(II) complexes of tridentate azo compounds. These are stable under only a very limited range of conditions and readily undergo oxidation to the corresponding 2 1 cobalt(III) complexes. 

The marked tendency for cobalt(II) to become oxidized to cobalt(III) after its extraction by hydroxyoximes is well known, and results from the large decrease in redox potential of the Co2+/Co3+ couple in the presence of nitrogen donor ligands... 

Table 10.1. Observed and calculated redox potentials of hexaamine cobalt(II/III) couples 132,133. ...

The redox potentials and the strain energies at the cobalt(III) and cobalt(II) oxidation states of die most stable conformers of a number of hexaaminecobalt(III/II) complexes are listed in Table 10.1. The strain energy difference between the two oxidation states was found to correlate with the experimentally determined reduction potential11331. Fig. 10.2 shows a plot of the redox potentials of the hexaaminecobalt(III/II) complexes from Table 10.1 as a function of die strain energy differences between the oxidized and reduced forms. The experimentally determined redox potentials are given as solid points while the line corresponds to the calculated potentials. Based on Eq. 10.1,... 

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