Importance and Applications of Coordination Compounds

Coordination compounds are far more than an abstract classroom topic: complex formation underlies practical chemical analysis, the industrial extraction of precious metals, and some of the most essential biochemical processes in living organisms.

In qualitative and quantitative chemical analysis, many metal ions are identified or estimated by the distinctive colour or precipitate they give on forming a specific complex. $\text{Ni}^{2+}$ is classically identified by adding dimethylglyoxime (DMG), which forms a bright red, chelated precipitate, $[\text{Ni}(\text{dmg})_2]$. $\text{Cu}^{2+}$ gives a pale blue aqua complex that turns a much deeper, diagnostic blue on adding excess ammonia, forming $[\text{Cu}(\text{NH}_3)_4]^{2+}$. $\text{Fe}^{3+}$ forms an intensely blood-red complex, $[\text{Fe}(\text{SCN})]^{2+}$, with thiocyanate ion, sensitive enough to detect even trace amounts of iron. EDTA, the hexadentate chelating ligand introduced in Section 5.2, is used quantitatively in complexometric titrations: because it forms an extremely stable 1:1 chelate with almost any metal ion of charge $+2$ or higher, a solution's total content of $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ — a direct measure of water hardness — can be determined by titrating against a standard EDTA solution, using an indicator that itself changes colour once free metal ion has been fully complexed.

In the extraction of metals, cyanide-complex formation is the industrial basis of the Mac Arthur-Forrest cyanide process, used to extract both gold and silver from low-grade ore. Finely crushed ore is treated with dilute aqueous sodium cyanide in the presence of air, which oxidizes and dissolves the metal as a soluble complex ion — for gold, $4\text{Au} + 8\text{CN}^- + \text{O}_2 + 2\text{H}_2\text{O} \to 4[\text{Au}(\text{CN})_2]^- + 4\text{OH}^-$. The dissolved gold is then recovered from this leach solution by adding metallic zinc dust, a more easily oxidized metal that displaces gold from its cyanide complex: $2[\text{Au}(\text{CN})_2]^- + \text{Zn} \to [\text{Zn}(\text{CN})_4]^{2-} + 2\text{Au}$. Without cyanide's ability to hold gold as a stable, soluble complex ion, this efficient wet-chemical extraction of gold from ore would not be possible at all. …