Anation cobalt complexes

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

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... 

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 ... 

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... 

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). 

Rate Constants for the Aquation of Pentaammine (ligand) cobalt(III) Complexes and Equilibrium Constants for the Anation of Pentaammineaquacobalt(III) with Various Anions... 

Two studies have been made of the hydroxo-bridge cleavage of the di-/u--hydroxobis[bis ethylenediamine cobalt(m)] complex (14). > Certain differences in the interpretation of the results are apparent. In perchloric acid, aquation and bridge cleavage take place the same seems to happen in nitric acid, but in hydrochloric or sulphuric acids, whose anions are potential ligands, there is some anation as well as aquation and bridge... 

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. ... 

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... 

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. 

Anation of irans-[M(L)(OH2)2] ions [L = mcso-tetrakis(p-sulfonato-phenyOporphyrinate ion M = Co, Rh, Cr] by NCS has been studied at high pressures. The volumes of activation and other activation parameters are compared in Table 6.8. Whereas the values of A5 would suggest a change in mechanism from dissociative for Co " to associative for Rh " and Cr ", the values of A V show that all three metals react dissociatively. The cobalt system is so labile that high-pressure stopped flow was needed to measure AV, and a D mechanism was found to operate in this case. The Rh " and Cr " complexes anate with an Id mechanism. ... 

The kinetics of anation of [Co(NH3)5(OH2)2] [and the analogous chromium(III) complex] by H3P03/H2P0 has been studied at different temperatures at 7= 1.0 mol dm. The results are interpreted in terms of an mechanism for the cobalt(III) complex. It is found that first-order rate constants for the neutral (ki) and anionic ( 2) species of the series H3P0 /H2P0 ( = 2-4) are the same for Cr(III), but with Co(III), the species H3PO is not reactive, except for n = 2. 

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