Preparation of coordination compounds
This chapter reviews the most common methods by which coordination compounds are prepared. However, current research is almost invariably aimed at producing the unusual and exotic, not the common. So, a contemporary research journal would describe methods
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S. F. A. Kettle, Physical Inorganic Chemistry © S. F. A. Kettle 1996
52
1
Preparation of coordination compounds
they undergo substitution reactions. When excess of aqueous ammonia is added to a solution of copper(II) sulfate in water the change in colour from pale to deep blue is almost instantaneous, because an ammine complex is formed very rapidly (in this reaction ammonia replaces some of the water molecules coordinated to the copper(II) ion). This is an example of the generalization that copper(II) forms kinetically labile complexes. On the other hand, it takes hours (or even days at room temperature) to replace water molecules coordinated to a chromium( III) ion by other ligands. Again we can generalize: chromium(III) forms kinetically inert complexes. It is important to recognize the distinction between kinetic and thermodynamic stability at this point. The thermodynamic stability of a complex (which will be discussed at length in the next chapter) refers to the concentrations of complex species and ligands at equilibrium. Kinetic stability refers to the speed at which equilibrium conditions are reached. As one would expect, the preparations of kinetically inert and labile complexes present quite different problems. In general, the ions of the second- and third-row transition elements usually form kinetically inert complexes. With the exception of chromium(III) and cobalt(III), the common ions of first-row transition elements usually form kinetically labile complexes. Metallic main group elements usually form labile complexes. Complexes involving low valence states, organometallic complexes for instance, are usually inert. However, inertness relates to kinetics and kinetics depend on mechanism. An organometallic compound which normally reacts slowly may spontaneously catch fire, or, less dramatically, rapidly oxidize, if exposed to air. Not surprisingly, special inert atmosphere techniques have to be used in preparing such compounds. Gaseous oxygen, of course, is a diradical, with two unpaired electrons, and so it is not unexpected that it should react rather differently to many other potential reactants.
4.2
Preparative methods
It is difficult to present reaction techniques in an order which is obviously logical and sequential. In the following pages the pattern usually adopted is to move from the simple to the complicated, although simplicity has its own complications. So, the first reaction considered is a simple gas-phase reaction between molecules, but one for which a quite complicated glass vacuum line would be needed. The reaction between aqueous Cu" and aqueous ammonia, considered later, can be carried out using a couple of test tubes, but is chemically quite complex.
4.2.1 Simple addition reactions The most direct method of preparing [BF3 (NH 3 )] is by gas-phase addition, in which a carefully controlled flow of each of the gaseous reactants is led into a large evacuated flask, where the product deposits as a white powder: BF3
+ NH 3
....
[BF3 (NH 3 )]
When one reactant is a liquid and t
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