Substitution reactions of octahedral

Kinetics and mechanisms of substitution reactions of octahedral macrocyclic amine complexes. C. K. Poon, Coord. Chem. Rev., 1973,10,1-35 (130). 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

For studies of substitution reactions of octahedral complexes, many... 

The Role of Ion Association in the Substitution Reactions of Octahedral Complexes in Nonaqueous Solution... 

The substitution reactions of octahedral metal complexes (Fig. 5-38) have been the subject of intensive investigation over the past forty years, with complexes of the non-labile ions such as chromium(m) and cobalt(m) playing a vital role in these studies. 

If the intermediates (or transition states) are drawn correctly in Figure 23-1, substitution by dissociation should convert the trails complex shown into another trans complex, but substitution by direct displacement should convert it partially (for there are paths other than the one shown) to a cis complex. Unfortunately, however, there is still some question concerning the geometry of such transition states, and stereochemical results do not often lead to straightforward mechanistic conclusions. Mechanistic information concerning substitution reactions of octahedral complexes may be obtained, but its interpretation is, in general, less direct than we would like. 

Recently, mechanistic studies have been extended to the substitution reactions of octahedral complexes of Pt(IV). It has been found, for example, that the organic base pyridine replaces chloride in the complex Pt(NH3)3ClJ at a rate proportional to the concentration of pyridine ... 

The steric course of substitution reactions of octahedral metal complexes other than cobalt (III) has been studied to a limited extent. Garner... 

On the other hand, Werner was aware of and did investigate substitution reactions of octahedral complexes which yield isomer mixtures (97-102). Furthermore, he knew that isomerization sometimes occurred without apparent reaction. In fact, his contemporary, S. M. J0rgensen, observed the isomerization of [CoCl2(en)2] before the turn of the century (62). Therefore, the stereomobility of substitution reactions of inert, octahedral species has long been of interest to coordination chemists. 

While this could be interpreted as a dissociative mechanism with an ion-pair preequilibrium, discussed above for octahedral complexes, it is notable that the similar substitution reaction of octahedral complexes that usually react by dissociative processes shows no dependence on entering group concentration. 

Stereochemical changes taking place in the course of substitution reactions of octahedral complexes have been reviewed. An improved method for estimating crystal field activation energies has been described. ... 

Throughout our discussion, we still have not answered the fundamental question What is the preferred mechanism of substitution reactions of octahedral coordination compounds The answer is that dissociative mechanisms are preferred. In this section, we investigate some of the evidence that supports such a conclusion. We cite three different types of reactions (1) the rates of exchange of water molecules, (2) anation reactions, and (3) aquation reactions. 

To conclude this section, we have seen thatvarious pieces of data from (1) rates of water exchange, (2) anations, and (3) various aquation reactions all seem to generally favor a dissociative mechanism for substitution reactions of octahedral compounds. 

Now that we have fairly well established that the dissociative mechanism generally applies for the substitution reactions of octahedral complexes, we are in a good position to begin to answer some of our earlier (p. 100) critical questions about inert versus labile complexes. As defined earlier,and inert-xro, kinetic terms describing the rates of reactions of coordination compounds. As you should recall from earlier courses, rates depend on the magnitude of the energy of activation, of the ratedetermining step. 

Although the overwhelming bulk of evidence for substitution reactions of octahedral complexes indicates that they most often proceed via a dissociative mechanism. 

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