Cobalt complexes water exchange reaction

We conclude with a consideration of a few other cobalt self-exchange reactions. The reaction in Eq. (9.33) is faster than that involving the ammine complexes (Eq. 9.30) because the water is a weaker-field ligand than ammonia. Thus, the activation energy for the formation of the electronically excited states is lower, as is the change in Co-ligand distances in the two oxidation states. 

The as-bromoamminebis(ethylenediamine)cobalt(III) bromide crystallizes in purple platelets. The compound has been resolved/ and the kinetics of the reactions of hydroxide ion with the d- and Z-isomers in water have been determined. The red-orange crystals of as-aquoammine-bis(ethylenediamine)cobalt(III) bromide are compact and almost cubic, in contrast to the orange needle-plates of the trans isomer. The crystals of water exchange of the aquoammine complexes have been studied. ... 

The rate enhancements observed in amine complexes of metal ions such as cobalt(III) and ruthenium(III) are not universally observed. Water exchange on [Fe(OH2)6l is more rapid in base ( 750-fold) (112), enhanced but less so for [Cr(OH2)6p ( 60-fold) (266), and absent in [V(OH2)6] (227). The trend reflects expectations of increasing associative character in reactions of these metal ions from Fe + to to eventually, the conjugate base may play no significant role in the exchange mechanism. For the type of complexes generally covered in this review, however, hydroxide ion causes a significant increase in lability. 

Furthermore, it is proposed that the active structures of Co " and Co " in anhydrous acetic acid are represented largely by uncharged sixfold-coordinated complexes such as Co (OAc)2(HOAc)4 and Co" (OAc)3(HOAc)3. An addition of water, substituted benzaldehydes, benzoic acids, or phenols might result in exchange reactions with acetic acid ligands, and influence the catalytic properties analogously to the effects observed upon addition of zirconium(IV) acetate [14w]. Thus, only at high cobalt(II) concentrations catalytically less active dimers will play a relevant role. 

The mechanism of 1 1 complex formation between palladium(II) and catechol and 4-methylcatechol has been studied in acidic media, and the rate of 1 1 (and 1 2) complex formation between silver(II) and several diols is an order of magnitude higher in basic solution than in acidic. The kinetics of formation and dissociation of the complex between cop-per(II) and cryptand (2,2,1) in aqueous DMSO have been measured and the dissociation rate constant, in particular, found to be strongly dependent upon water concentration. The kinetics of the formation of the zinc(II) and mercury(II) complexes of 2-methyl-2-(2-pyridyl)thiazolidine have been measured, as they have for the metal exchange reaction between Cu " and the nitrilotriacetate complexes of cobalt(II) and lead(II). Two pathways are observed for ligand transfer between Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) and their dithiocarbamate complexes in DMSO the first involves dissociation of the ligand from the complex followed by substitution at the metal ion, while the second involves direct electrophilic attack by the metal ion on the dithiocarbamate complex. As expected, the relative importance of the pathways depends on the stability of the complex and the lability and electrophilic character of the metal ion. 

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... 

It has been concluded from a study of the optical and e.p.r. spectra of Co —Cu bovine superoxide dismutase, in which zinc has been replaced by cobalt, that the cobalt site reactivity should be described in terms of reaction of the Co-imidazolate-Cu system as a whole the crystal structure reported last year indicated that the metals were linked by a common histidine residue. There is an exchange interaction between the cobalt and copper however, this is abolished when the linking imidazole is protonated. Further evidence for the close proximity and interactive dependence of the zinc and copper binding sites was obtained from a study of the 4 Cu protein a two-fold enhancement of the activity of 2 Cu dismutase was observed upon occupation of the zinc sites by the Cu ". On the basis of C1 n.m.r. studies, Fee and Ward have suggested that one co-ordination position of Cu in superoxide dismutase is normally occupied by water they further suggest that superoxide can displace the solvent to form a cupric peroxide complex. 

Cobalt is found in vitamin Bn, its only apparent biological site. The vitamin is a cyano complex, but a methyl or methylene group replaces CN in native enzymes. Vitamin-Bi2 deficiency causes the severe disease of pernicious anemia in humans, which indicates the critical role of cobalt. The most common type of reaction in which cobalamin enzymes participate results in the reciprocal exchange of hydrogen atoms if they are on adjacent carbon atoms, yet not with hydrogen in solvent water ... 

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