The co-ordination compounds are of great importance. These compounds constitute the minerals, plants and are also present in animals. These play important functions. For example, chlorophyll (present in plants) are both co-ordination compounds and are of great importance in regulating the various biological functions of plants and animals. Besides these, the co-ordination compounds also find a wider application in the fields of metallurgy, biochemistry, water softening, ion exchange, electrochemistry, textile dyeing, bacteriology and analytical chemistry. For example, EDTA and DMG are required to precipitate metals like Ni, Zn, Cd, etc. apart from these; the study of complex compounds has enlarged our understanding of chemical bonding, and certain physical properties such as spectral, magnetic properties, etc.
A large variety of co-ordination complexes both naturally existing as well as synthetically prepared are known to us. As the central metal ion is surrounded by different molecular species, these complexes possess different physio-chemical properties and find many applications in different processes. A few applications of the co-ordination complexes are summarised below:
1. Qualitative analysis: the formation of complex substances by using suitable reagents is very effectively used in the separation and detection of cation in qualitative analysis. For example, Group I of the scheme for qualitative analysis has Ag+, Pb+2 and Hg22+. All these are first precipitated as chlorides. PbCl2(s) is removed from AgCl(s) and Hg2Cl2(s) by its greater solubility in hot water. AgCl(s) is separated from Hg2Cl2(s) by its solubility in NH3(aq) with which it forms a soluble complex
AgCl(s) + 2NH3(aq) 
[Ag(NH3)2]+(aq) + Cl-(aq)
Similarly, the separation of Cd+2 from Cu+2 could also be achieved by treating the solution of Cu+2 and Cd2+ will excess of CN-. The following reaction occurs
Cd+2 + 4CN- [Cd(CN)4]2- ….. kf = 7.1 × 1018
2Cu+2 + 8 CN- 2[Cu(CN)3]2- + C2N2(g) …… kf = 2 × 1027
Now, when the solution is saturated with H2S, the ionic product does not exceed the Ksp of Cu2S due to formation of a highly stable complex of copper and hence only CdS gets precipitated.
Nickel ions are estimated by forming complex with dimethylglyoxime (DMG).
Ca+2 and Mg+2 ions are estimated as their complexes with EDTA.
2. Dissolution of insoluble compounds: by complex formation, water insoluble species are brought into solution. For example, in red bauxite, Al2O3 is separated from Fe2O3 by heating with concentrated sodium hydroxide solution. Al2O3 dissolves due to the formation ofAl(OH4)- complex ion.
Al2O3(s) + 3H2O(l) + 2OH-(aq) 2Al(OH)4-(aq)
3. In photography, developed films are fixed by washing it with a solution of sodium thiosulphate.
AgBr(s) + 3Na2S2O3(aq) Na3[Ag(S2O3)2](aq) + NaBr(aq)
4. The nature of ligands: changes the redox behaviour of the metal ions. For example, in the presence of some ligands cobalt is easily oxidized from +2 oxidation state to +3 oxidation state.
5. Estimation of Hardness of water: the hardness of water is estimated by simple titration against EDTA solution. EDTA forms stable complexes with the metal ions present in the hard water. As stability constants of calcium and magnesium complexes of EDTA are different, even the selective estimation of these ions is possible.
6. Electroplating: many co-ordination complexes are used as electrolytes for electroplating. These complexes deliver the metal ions in controlled manner. For example, for silver plating the complex K[Ag(CN)2] is used.
7. Extraction of metals: silver and gold are extracted from their respective ores by treatment with sodium cyanide solution. This also involves complex formation.
Ag+(aq) + 2NaCN(aq) Na[Ag(CN)2](aq) + Na+
Au+(aq) + 2NaCN(aq) Na[Au(CN)2](aq) + Na+
8. Biological used
(i) The complex of calcium with EDTA is used to treat lead poisoning. Internal to the body calcium in the complex is replaced by lead. The more stable Pb-EDTA complex is estimated in urine.
(ii) The platinum complex cis – [Pt(NH3)2Cl2] known as cisplatin is used as an antitumor agent in treatment of cancer.
(iii) Many natural compounds exist as co-ordination complexes. For example, haemoglobin (a complex of Fe2+), chlorophyll (a complex of Mg2+) and vitamin B12 (a complex of CO2+).
9. Plant growth: plants need various nutrients for healthy growth. The essential nutrients include a number of metals such as iron, zinc, copper, manganese and molybdenum. Iron in its +3 oxidation state in the soil is mostly hydrolysed to form insoluble iron hydroxides such as Fe(OH)3 which cannot be taken up by plants. Thus due to deficiency of iron plants develop a disorder known as iron chlorosis, evidenced by yellowing leaves.
The standard treatment for this problem is supplying Fe (III) – EDTA complex. This complex is soluble in water and readily enters the roots of trees, where it is eventually converted into a utilizable form.
10. In living systems: coordination compounds play many important roles in animals and plants. They are essential in storage and transport of oxygen, as electron transfer agents, as catalysts and in photosynthesis.
Haemoglobin, a protein present in blood serves the purpose of carrying oxygen in the blood from the lungs to tissues. Porphine molecule is a significant part of haemoglobin structure.
Complexes resulting from porphine are called porphyrins. In the structure the metal ions can replace the two H atoms on the central N atoms and co-ordinate simultaneously with all four N atoms. The porphyrin is thus a tetradentate ligand or chelating agent for the central metal ion.