Cobalt anation reactions

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... 

The most obvious examples are in the preparation of octahedral Co(III) complexes. The most readily-available cobalt compounds are Co(II) salts in the presence of suitable ligands - usually N-donors - these are oxidised to give Co(III) complexes by air or hydrogen peroxide. A few such easily-prepared complexes open up pathways to the vast number of known octahedral Co(III) complexes via substitution reactions. For example, [Co(NH3)5(H20)]CI3 is readily converted into [Co(NH3)5X]Cl2 via anation reactions of the type discussed in Section 9.5, and salts containing the [Co(NH3)4(C03)]+ ion (where the carbonate is bidentate, taking up two cis positions) are useful for the formation of cw-[Co(NH3)4X2]+. 

Kinetically, there are inherent difficulties in distinguishing the three mechanisms. As an illustration, is the anation reactions of the inert (d low-spin) complexes of cobalt(III), as typified by... 

Reviews of interest include a general review of anation reactions of cobalt(III) complexes and a discussion of the solvation of transition metal complexes/ The nature of the solvent can have a very large effect on such properties as solubilities, reactivities, redox potentials, formation constants, and various types of spectra. Such solvent effects reflect changes in the solvation of ions, complexes, initial states, transition states, and excited states. 

Cobalt(III) forms a wide range of substitution-inert (low-spin d ) complexes whose thermal aquation and anation reactions have been thoroughly studied. These provide useful comparisons for photochemical... 

Cobalt(m).— Anation of [Co(NH3)6(OH2>] + by sulphate or chloride proceeds by a dissociative interchange mechanism, in which the reactive species are ion-pairs and ion-triplets, The activation enthalpy for anation of [Co(NH3)6(OHa)] + by glycine is 29 kcal mol again a dissociative mechanism is operative. An earlier study of oxalate anation of [Co(en)a(OH2)2] in acidic solution is complemented by a study of oxalate anation of [Co(en)a(OH2)(OH)] + in basic solution. Several examples of anation reactions of bridged dicobalt complexes have been mentioned in the section on aquation of these complexes. ... 

The anation reaction of a cobalt(III) ammine aquo complex using (N02 ) as an anionic ligand also occurs without the cleavage of metal-ligand bond. 

Pentaaniinineaquacobalt(III) halides are relatively unstable. Substitution of coordinated water by halide anation [19]) is detectable at about 360 K. Conductivity measiuements have shown [19] that the water evolved may remain temporarily as a liquid before evaporation and this endothermic effect may mask the exothermic anation. This is consistent with the observation [4] that the minimum temperatures of cobalt reduction in the reactions of [Co(NH3)5H20]X3 (X = Cl, Br and I ) were very close to those of the corresponding halopentaammines. 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Cobalt(lII) (High-pressure studies of the anation of [M(NH3)5(OH2)] + ions (M = Co, Rh, or Cr) gave the AF+ values shown in Table 1. These results, together with those for the corresponding aquation reactions enable values for A to be estimated, and the values obtained... 

Although all of these examples involve cobalt(III) complexes, reactions involving other metal ions could be cited as examples. This paper will deal mainly with the complexes of cobalt(III) and chromium(III) and will be limited to the anation, racemization, and geometric isomerization processes. Linkage isomerization will not be discussed since such reactions are fairly familiar and have been recently reviewed elsewhere. " Ligand exchange will be discussed only as it is encountered in racemization and geometric isomerization. 

Anation of the water-soluble porphyrin complex [Co(P)(OH2)2l [H2P = 5,10,15,20-tetrakis-(A -methylpyridyl)porphine] is very fast for a substitution reaction at cobalt(ra). Rate data are also reported for the anation of the [Co(P)(NCS)-(0H2)] + and [Cb(P)(NCS)(OH)] + ions, and thiocyanate ion is shown to have a strong trans effect (> 10 ) upon the remaining co-ordinated water molecule. Omitting charges for convenience, rate data for the following reactions were as shown ... 

A consideration of solvent effects on rates of formation reactions of the rran -[Rh(dmgH)2(OH2)2] cation in binary aqueous solvent mixtures in terms of initial state and transition state components is included in the paper on formation reactions of aquo-cobalt(III) complexes and of Niaq dealt with fully in Section 5.7.1.3. However it is perhaps proper to point out here the contrast between the D mechanism assumed in this analysis of reactivity trends and the associative mechanism proposed for anation at the very similar species [Rh(tpps)(OH2)2] (see Section 5.8.4). 

It has been known for a few years that anation of /m -[Co(dmgH)2-(Me)(OHa)] is fast, since the methyl group has a powerful trans labilizing influence on the co-ordinated water. Recently activation parameters have been determined for replacement of this water by pyridine and by thiocyanate. Activation enthalpies are low by cobalt(m) standards 20.5 kcal mol for reaction with pyridine and 17.5 kcal mol for thiocyanate. However, these values are comparable with those for anation of the analogous, but much less reactive, complex /rans -[Co(dmgH)2(N02)-(0H2)] in fact it is the difference in activation entropies which determines the different reaction rates for these methyl and nitro-complexes. The... 

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