Cobalt outer-sphere coordination

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

Both inner- and outer-sphere electron transfer mechanisms will be investigated in this series of experiments. The chapter begins the synthesis of four cobalt(ffl) coordination complexes followed by analysis and reactivity studies. Electronic structure will be investigated using visible spectroscopy and the redox chemistry of two of the complexes will be examined... 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

The [Co ] /[Co(sep)] " reactions are formally three-electron processes, similar to those discussed above, while the [Co ] " /[Ru(NH3)6] reactions are two-electron processes. The magnetic field and IPCT perturbation studies indicate that ei < 1 for both of these classes of electron transfer reaction, but smallest for the former. For a given coordination environment, outer-sphere electron transfer reactions involving cobalt(III) and high-spin cobalt(II) couples are likely to result in smaller values of than most other transition metal, one-electron transfer couples. 

The oxidation of [Co(CN) 3 to cobalt(III) is accompanied by an increase in the coordination number of the cobalt ion. The mechanism of electron transfer may be inner- or outer sphere, depending on whether the sixth ligand required to complete the octahedral coordination shell of the cobalt ion is derived directly from the oxidant or, alternatively, is an anion (e.g, CN ) derived from the solution. The substitution-inertness of the product, and the different kinetic patterns of the two paths, permit this ligand to be identified and the mechanism of electron transfer to be established. 

There have been a few reports over the years on nitrate catalysis of substitution at cobalt(III) and at chromium(III). Now it has been shown that nitrate has a catalytic effect on oxalate anation of a-c/5-[Co(edda)(OH2)2] - The outer-sphere association constant appears to be larger when nitrate is present. The possibility of the formation of a transient five-coordinate intermediate under the influence of the nitrate (cf. the end of the previous section, 5.7.3.) should not be ruled out. ... 

In general, two distinct types of processes have been recognized, due to the fact that the ions in solution are almost always surrounded by two layers of solvent molecules. In particular, in the case of a transition element such as cobalt, the first hydration sphere or inner shell corresponds to ligands fixed by coordination bonds and the second, outer sphere includes solvation molecules fixed less rigidly by electrostatic attraction. If, in the active complex, only the outer spheres overlap, the process takes place by the outer sphere , but if the inner spheres overlap, it takes place by the inner coordination sphere . 

Ionization isomerism is best characteized in systems involving substitutionally inert metal ions where kinetic, rather than thermodynamic, factors control isomer formation. Numerous cases are known of cobalt(III) compounds in which exchange of a ligand between inner and outer coordination spheres results in a spectacular colour change. Thus, tra/is-[CoCl2(en)2]N02 (en = 1,2-ethanediamine) is green and frans-[CoCl(N02)(en)2]Cl is orange tmns-[CoCl(NCS)-(en)2]NCS is blue and frans-[Co(NCS)2(en)2]Cl is deep red. 

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