Cobalt redox with complexes

The dark green complex [Co PhB(Bu lm)3 NHBu ] can further react with a radical proton abstractor (a phenol radical) to form the corresponding cobalt(Ill)-imido complex in a redox reaction that is reported to model a cracial step in the oxidation of water to elemental oxygen in the catalytic system of photosystem II [406] (see Figure 3.131). A similar iron(III) complex exists as well [415]. 

Two perfectly reversible one-electron reduction steps are observed for Co.S ", at —0.53 V (Co +/+) and 1.205 V (Co+/ ). Comparison of the electrochemical properties of the cobalt catenate with the previously reported data for cobalt bpy or phen complexes [31, 32] shows here again a strong stabilization of the reduced states, Co and Co". Other sterically constrained polyimine ligands also lead to stabilized monovalent cobalt complexes [33-37]. Remarkable also is the drastic structural effect of the catenate on the Co +/-+ redox potential whereas bpy or phen complexes of cobalt(II) can easily be oxidized to octahedral cobalt(III) [36, 38], the redox potential value of the Co + -+ couple being close to 0 V, no oxidation peak is observed for Co.5 + prior to ligand oxidation (Ep > 1.6 V). The destabilization effect of Co " due to tetrahedral environment provided by the entwined and interlocked structure of 5 is thus very large (>1.5 V). 

The simple but rather useful molecular mechanics calculations were applied to both electron transfer kinetics and reduction potentials for a wide range of hexamine cobalt(III/II) complexes with primary, secondary, tertiary, and macrobicyclic amine ligands [237]. The redox potentials of the Co +/2+ couples varied from... 

The redox properties of complexes of the cobalt triade with [2 ]cyclophanes [78] as well as iron [180] were also examined and show a similar behaviour as... 

Reaction 3. This reaction has been described as a "Redox-Switch" mechanism. The metal ion in its lower oxidation state first forms an "outer sphere" complex with the corrin macrocycle, followed by a two electron transfer of the complexed metal ion to a two electron acceptor. Oxidation of the complexed metal ion facilitates carbanion transfer from the cobalt to the complexed metal. 

Cobalt(II) (F).— The reaction of cobalt(ii) with pan (5) is slightly slower than that with par (6), allegedly owing to the greater bulk of the pan ligand. Several complex formation reactions show complicated kinetic patterns due to parallel redox processes. This is true for the cobalt(ii) reactions mentioned above ... 

A somewhat simpler rate law is observedjji oxidation of [Cofedta)] -, In this system there is kinetic evidence for an inner-sphere process involving the complex [(edta)Co i—which undergoes an intramolecular redox transfer with the formation of a cobalt(ra)-edta complex which ring-closes to the sexadentate [Co(edta)] ion. 

Several other useful reviews of reactions involving metal ions have also been published. Redox reactions of chromium(m)-amine species have been described and a survey has been made of the solution chemistry together with reaction paths involved in the redox reactions of various plutonium species. Oxidation reactions of thallium(m) have also been described. Developments in the redox chemistry of peroxides have been reviewed, the nature of the reactions which involve iron(iii) in various complexed forms providing a fascinating example of the manner in which geometry and co-ordination to the metal centre greatly affect the reactivity of the system. Redox properties of cobalt chelates, with delocalized... 

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