Coordination compounds substitution reactions

Mercuration and chloromethylation reactions as well as halogena-tion seem to proceed with preliminary coordination followed by substitution in the coordination compound. Such reactions as nitration and sulfonation in concentrated acids appear to proceed differently as evidenced by the substitution of the phenyl nucleus on nitration and by the sulfonation of phenylisoxazolcs. 

The importance of substitution reactions cannot be overstated. Systematic investigations of coordination compound substitution kinetics, and mechanisms shed light on the electronic structure of compounds and on their interactions. Although formally taken up in Chapter 4, substitution is encountered in all chapters of this book. 

Five different types of reactions of coordination compounds substitution, dissociation, addition, redox or electron transfer, and the reaction of a coordinated ligand. 

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

Brand-new results show the existence of heptacoordinated silicon as described in some of the following papers of this chapter, which also contribute to the discussion of mechanistical pathways in the course of nucleophilic substitution reactions at silicon. From these results one may speculate whether compounds with octa- and nonacoordinated silicon may be characterised in the near future. Although it is a problem to assign coordination numbers in -w-bound systems, it is worthwhile to note Jutzi s dccamethylsilicocene with a formal Si-coordination number ten in the oxidation state +2 in this context. With respect to Si(U)-compounds it should be stated that there are further derivatives with the... 

Compounds with High Coordination Numbers at Silicon Models for the Investigation of the Nucleophilic Substitution Reaction at Silicon Centers... 

Addition reactions to coordinatively unsaturated compounds are expected to be significantly faster than substitution reactions. 

The cationic dinuclear oxo-gold(m) complexes of substituted bipyridyls react with olefins to give the stable coordination compounds shown in Scheme 79. The reactions are often incomplete, produce many byproducts, and give low yields of the olefin complexes/... 

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... 

The most common reaction exhibited by coordination compounds is ligand substitution. Part of this chapter has been devoted to describing these reactions and the factors that affect their rates. In the solid state, the most common reaction of a coordination compound occurs when the compound is heated and a volatile ligand is driven off. When this occurs, another electron pair donor attaches at the vacant site. The donor may be an anion from outside the coordination sphere or it may be some other ligand that changes bonding mode. When the reaction involves an anion entering the coordination sphere of the metal, the reaction is known as anation. One type of anation reaction that has been extensively studied is illustrated by the equation... 

Much of Chapter 20 was devoted to the description of mechanisms of reactions of coordination compounds with emphasis on substitution reactions. Because there are numerous aspects of oxad reactions that are different from those involving substitution, we will address some of the mechanistic aspects of oxad reactions briefly in this section. 

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

---