Coordination compounds ligand replacement reaction

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... 

Thus five steps are involved in the reaction between a hexaaquachromium(III) ion and an o,o -dihydroxydiarylazo compound at low pH to form the 1 1 chromium complex dyestuff three ligand replacement reactions and two ionization stages. Studies with other ligands lead to the conclusion that the probable rate-determining step is replacement of the first of the six coordinated water molecules in the highly symmetrical hexaaquachromium(III) ion by one of the donor functions of the ligand. For example, it has been shown that the rate of reaction between the hexaaquachromium(III) ion and the bidentate oxalate or malonate ions to form monochelate... 

Mixed phosphite ester-phosphine coordinated acylcobalt carbonyl derivatives have also been prepared. Methyl(triphenylphosphine)cobalt tricarbonyl, which cannot be produced by a simple ligand replacement reaction, can be obtained by another method. Hieber and Lindner (21) found that bis(triphenylphosphine)dicobalt hexacarbonyl reacts with sodium amalgam to form sodium(triphenylphosphine)cobalt tricarbonyl and that this compound reacted with methyl iodide to form methyl-(triphenylphosphine)cobalt tricarbonyl. 

The replacement of one ligand by another is the most common type of reaction of coordination compounds, and the number of reactions of this type is enormous. Some are carried out in aqueous solutions, some in nonaqueous media, and others can be carried out in the gas phase. One such reaction is... 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

Of all the types of reactions known for coordination compounds, the most thoroughly studied are those known as substitution reactions, in which one ligand replaces another. 

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