Substitution reactions of octahedral complexes

Now the concentration of the total complex, [C], is the sum of that which is free, [C], and that which is in the encounter complex, [CY]  

It can immediately be seen that when the product [Y] is small compared to 1 then the commonly observed proportionality of the rate to both [C]t and [Y] is explained. It transpires that the—often unexpectedly small— variations in rate constant mentioned above are the consequence of changes in K, not of the intrinsic rate k. So, for nickel(II) complexes the observed formation rates with respect to [Ni(H20)6] vary from about 200 to 7000000 s a factor of over 30000, but the corresponding k values 

Sr Ca Mg Be and so on it is also usually true that for a given ionic size an increase in formal charge is associated with a decrease in reaction rate. The observed tendency of a change from dissociative to associative activation on moving down a Group in the Periodic Table becomes, therefore, rather more understandable. 

6 Base-catalysed hydrolysis of cobalt(lll) ammine complexes 

This pre-equilibrium is well over to the left-hand side and is rapidly established. However, the deprotonated species is up to ca. 10 times as reactive as the species from which it is derived. 

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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... 

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. 

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. 

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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