Importance and Applications of Coordination Compounds

Coordination compounds are not just a theoretical curiosity — they occur naturally across the mineral, plant and animal worlds, and they are put to deliberate use across analytical chemistry, metallurgy, biology, industry and medicine.

Use in Qualitative and Quantitative Analysis

Many familiar colour reactions used to detect and estimate metal ions are really coordination chemistry at work. When a metal ion combines with a suitable ligand — especially a chelating ligand — the coordination entity formed often has a characteristic colour or property that classical and instrumental methods of analysis can exploit. Reagents commonly used for this purpose include EDTA, DMG (dimethylglyoxime), $\alpha$-nitroso-$\beta$-naphthol, and cupron.

A practical example of this analytical use is estimating the hardness of water. Hard water contains $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions, and their amount can be found by simple titration against $\text{Na}_2\text{EDTA}$. Both ions form stable complexes with EDTA, but their stability constants differ, which allows the two ions to be estimated selectively rather than together.

Extraction and Purification of Metals

Complex formation is central to how certain metals are extracted and purified.

• Extraction of silver and gold: these processes rely on forming a complex with the metal. Gold, for instance, reacts with cyanide in the presence of oxygen and water to give the coordination entity $[\text{Au(CN)}_2]^-$ in aqueous solution. Once gold is held in this soluble complex form, it can be recovered as the metal by adding zinc, which displaces it back to its metallic state.
• Purification of metals: a metal can also be purified by first converting it into a coordination compound and then decomposing that compound to release the pure metal. Impure nickel, for example, is converted to $[\text{Ni(CO)}_4]$, and decomposing this compound afterward yields nickel in its pure form.

Role in Biological Systems

A number of processes essential to life depend on coordination compounds built around a central metal ion:

• Chlorophyll, the pigment responsible for photosynthesis, is a coordination compound of magnesium.
• Haemoglobin, the red pigment of blood that carries oxygen, is a coordination compound of iron.
• Vitamin $B_{12}$ (cyanocobalamin), the factor that prevents pernicious anaemia, is a coordination compound of cobalt.
• Certain enzymes that act as biological catalysts also depend on coordinated metal ions — examples include carboxypeptidase A and carbonic anhydrase.

Industrial Catalysis

Coordination compounds serve as catalysts in a range of industrial processes. A well-known example is the rhodium complex $[(\text{Ph}_3\text{P})_3\text{RhCl}]$, known as Wilkinson's catalyst, which is used to hydrogenate alkenes.

Electroplating

Metal ions in solution as simple ions do not always plate evenly. Articles can be electroplated with silver and gold far more smoothly and evenly when the plating bath uses solutions of the complexes $[\text{Ag(CN)}_2]^-$ and $[\text{Au(CN)}_2]^-$ rather than solutions of the plain metal ions.

Photography

In black-and-white photography, once the film has been developed, it still carries undecomposed silver bromide that must be removed. This is done by washing the film with hypo solution, which dissolves the leftover $\text{AgBr}$ by converting it into the soluble complex ion $[\text{Ag(S}_2\text{O}_3)_2]^{3-}$, fixing the image.

Medicinal Applications …