Substitution reactions in octahedral complexes

We have barely introduced the vast area of synthetic coordination chemistry. The methods described show that a wide variety of techniques have been employed, and they may be employed in combination in sequential steps to develop creative synthetic routes. The comprehensive compilations listed in the suggested readings should be consulted for further study. Several journals in inorganic chemistry are at least partially devoted to synthesis of materials, so the field is growing at a rapid rate. 

The enormous number of coordination compounds undergo many reactions, but a large number of reactions can be classified into a small number of reaction types. When one ligand replaces another, the reaction is called a substitution reaction. For example, when ammonia is added to an aqueous solution containing Cu2+, water molecules in the coordination sphere of the Cu2+ are replaced by molecules of NH3. Ligands are held to metal ions because they are electron pair donors (Lewis bases). Lewis bases are nucleophiles (see Chapter 9), so the substitution of one nucleophile for another is a nucleophilic substitution reaction. Such a reaction can be illustrated as 

Because all of the ligands are leaving one metal ion and attaching to another, this type of reaction is sometimes known as ligand scrambling. 

Nucleophilic substitution reactions have rates that vary enormously. For example, the reaction 

Although the terms labile and inert have been in use for more than 50 years, they are only qualitative descriptions of substitution rates. A more appropriate way to describe the rates has been given by Gray and Langford (1968), which categorizes metal ions according to the rate of exchange of coordinated water with water in the bulk solvent. The four classes of metal ions are shown in Table 20.1. 

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Substitution reactions in octahedral complexes may proceed by D, Idt Ia, or A mechanisms and it is often difficult to distinguish between them because the rate law by itself does not allow the distinction to be made. Consider the replacement of water by ligand L under neutral conditions ... 

Figure 5-38. The prototypical ligand substitution reaction in octahedral complexes. In principle, the reaction could proceed by associative or dissociative mechanisms.

One strong indication of the importance of ligand field effects is the fact that substitution reactions in octahedral complexes of Pt4+, Rh3+, and Ru3+ occur without rearrangement. The following are examples of reactions of this type ... 

Although, as indicated in the preceding discussion, the majority of substitution reactions in octahedral complexes appear to proceed by an essentially dissociative pathway, there may be exceptions. 

Substitution reactions in octahedral complexes follow either substitution nucleophihc unimolecular (SN [1]) or substitution nucleophUic bimolecular mechanism. These pathways involve five coordinated (trigonal bipyramidal/ square pyramidal) and seven coordinated (pentagonal bipyramidal) intermediates, respectively. The formation of any of these intermediates involves the lowering of the symmetry. This, in turn, reduces the crystal field stabihzation energy (CFSE). This loss in CFSE is the activation energy (E required for the transformation. The octahedral complexes formed by the ions, for which there is a large loss in CFSE, do not react rapidly and are inert. However, the complexes of the ions for which there is httle or no loss in CFSE are labile. 

Mechanisms of Nucleophilic Substitution Reactions in Octahedral Complexes 141... 

MECHANISMS OF NUCLEOPHILIC SUBSTITUTION REACTIONS IN 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)... 

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. 

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. 

Nucleophilic substitution (SN) reactions in octahedral complexes are of paramount importance in coordination chemistry. These reactions follow any of the two popular mechanisms SN and SN. ... 

Consider a nucleophilic substitution reaction in a complex with coordination number six (CN = 6) octahedral (Oh) substrate MX5Y occurring via SN mechanism. 

The d metal ions, such as Pt(II), Pd(II) and Ni(II), often fonai square planar complexes. The square planar complexes of Pt(II) are of particular interest in kinetic studies due to their high stability, ease of synthesis and moderate rates of reaction that enable the monitoring of the reaction. The area of discussion in these complexes is restricted only to the substitution reactions. As compared to the octahedral complexes, the crowding around the metal ion is less in square planar complexes. This is one of the important reasons that most of the substitution reactions in these complexes follow the SN (associative mechanism). 

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