Reactions of octahedral complexes

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... 

As with solvolysis reactions of octahedral complexes, the rate-determining step may be solvolytic or dissociative in any case, it is independent of the concentration of Y ... 

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

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 ... 

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. 

The trans Effect. This is a particular feature of ligand-replacement reactions in square complexes which is of less importance in reactions of octahedral complexes except in some special cases where CO (or NO) is present as a ligand, or where M==0 or M=N bonds are present (see, e.g., Section 26-D-9). Most work has been done with Pt11 complexes, which are numerous and varied and have fairly convenient rates of reaction. Consider the general reaction (21-28) ... 

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. ... 

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. 

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