Importance and applications of coordination compounds
NCERT Class 12 Chemistry Chapter 5 Section 5.7
Quick answer
Coordination compounds support metal-ion analysis, metal extraction, catalysis and biological functions. Examples include magnesium in chlorophyll, iron in haemoglobin, cobalt in vitamin B12 and complexes used in water-hardness titrations. Some have medical uses, including specific chelating-drug formulations and cisplatin; the chemical form matters.
With your textbook
NCERT Chemistry Part I (Class 12), Chapter 5 “Coordination Compounds”, Section 5.7 “Importance and Applications of Coordination Compounds”, pages 136–137.
Open the official NCERT chapter (PDF, 3.7 MB, ncert.nic.in) (opens in a new tab)
Checked against the 2026-27 reprint on .
Why are coordination compounds important?
Importance of coordination compounds
Coordination compounds matter far beyond the laboratory: they detect and estimate metal ions in analysis, extract and purify metals, carry out life processes and serve industry and medicine. NCERT puts it this way:
These compounds are widely present in the mineral, plant and animal worlds and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine.
NCERT Chemistry Part I (Class 12), Section 5.7, page 136
In plain words: whenever a metal ion has to be held tightly, carried, hidden or picked out from others, chemists and living things wrap it in ligands. Most uses below come down to forming a stable complex, often with a chelating ligand.
Qualitative and quantitative analysis
- Many metal ions give characteristic colours with ligands, especially chelating ligands, because they form coordination entities. These colour reactions are the basis of detecting and estimating the ions by classical and instrumental methods (NCERT page 136).
- Reagents of this kind include EDTA, DMG (dimethylglyoxime), α-nitroso-β-naphthol and cupron (NCERT page 136).
Hardness of water
- The hardness of water is estimated by a simple titration with Na2EDTA: the Ca2+ and Mg2+ ions form stable complexes with EDTA (NCERT page 136).
- The two ions can be estimated selectively using controlled conditions, including pH and suitable indicators, that exploit differences in their conditional complex stability (NCERT page 136).
- The calcium and magnesium EDTA complexes used in this titration have a 1 : 1 metal-to-ligand ratio: the moles of EDTA used equal the moles of Ca2+ plus Mg2+ (NCERT pages 121 and 136). Hexadenticity describes six donor contacts, and does not by itself prove the stoichiometry of every complex. The titration also requires suitable pH, an endpoint indicator and control of interfering ions.
Extraction of metals
- Silver and gold. Some extraction processes use complex formation. Gold combines with cyanide in the presence of oxygen and water to form [Au(CN)2]− in solution, and metallic gold is then recovered by adding zinc (NCERT page 136).
- Purification. Metals can be purified by forming a coordination compound and then decomposing it: impure nickel is converted to [Ni(CO)4], which is decomposed to give pure nickel (NCERT pages 136–137). Balanced: Ni + 4CO → [Ni(CO)4], and on decomposition [Ni(CO)4] → Ni + 4CO (reasoned).
Biological systems
Coordination compounds are of great importance in biological systems (NCERT page 137):
| Compound | Metal | Role |
|---|---|---|
| Chlorophyll | Mg | photosynthetic pigment |
| Haemoglobin | Fe | oxygen carrier in blood |
| Vitamin B12 (including cyanocobalamin) | Co | anti-pernicious anaemia factor |
- Enzymes with coordinated metal ions, such as carboxypeptidase A and carbonic anhydrase, are the catalysts of biological systems (NCERT page 137). NCERT Class 11 Biology names the metal in one of them: zinc is a cofactor for the proteolytic enzyme carboxypeptidase (NCERT Class 11 Biology, Chapter 9, page 118).
Industry and medicine
- Catalysts. Coordination compounds catalyse many industrial processes; Wilkinson’s catalyst, the rhodium complex [(Ph3P)3RhCl], is used to hydrogenate alkenes (NCERT page 137).
- Electroplating. Silver and gold plate much more smoothly and evenly from solutions of [Ag(CN)2]− and [Au(CN)2]− than from solutions of the simple metal ions (NCERT page 137).
- Photography. In black and white photography the developed film is fixed with hypo solution, which dissolves the undecomposed AgBr as the complex ion [Ag(S2O3)2]3− (NCERT page 137).
- Chelate therapy. Metals present in toxic amounts are removed by chelating ligands: excess copper and iron with the chelating ligands D-penicillamine and desferrioxamine B (NCERT prints “desferrioxime B”: see the slip below), and lead poisoning with EDTA (NCERT page 137). Beyond the NCERT text: D-penicillamine is the one for copper, a chelating agent for removing excess copper in Wilson’s disease (DailyMed, Cuprimine label), and desferrioxamine is the one for iron (see the slip below); the EDTA salt used for lead poisoning is edetate calcium disodium (DailyMed, US National Library of Medicine).
- NCERT slip (page 137): NCERT prints “desferrioxime B”. Beyond the NCERT text: the iron-binding drug is desferrioxamine, also called deferoxamine (Wikipedia, “Deferoxamine”). Recognise the textbook spelling, but use desferrioxamine or deferoxamine for the drug.
- Cancer treatment. Some platinum complexes inhibit the growth of tumours; for example cis-platin and related compounds (NCERT page 137).
Solved examples
Example 1: Fixing a photograph
Write a balanced equation for the dissolution of AgBr in hypo (sodium thiosulphate, Na2S2O3) and give the oxidation state of silver in the complex.
Answer: AgBr + 2Na2S2O3 → Na3[Ag(S2O3)2] + NaBr. In [Ag(S2O3)2]3−, x + 2(−2) = −3, so silver is +1 (complex ion from NCERT page 137; equation reasoned).
Example 2: Estimating hardness
50.0 mL of a water sample needs 10.0 mL of 0.0100 M Na2EDTA. Find the total concentration of Ca2+ and Mg2+.
Answer: Moles of EDTA = 0.0100 × 0.0100 L = 1.00 × 10−4 mol, and EDTA binds the ions 1 : 1, so (Ca2+ + Mg2+) = 1.00 × 10−4 mol ÷ 0.0500 L = 2.00 × 10−3 mol L−1 (reasoned).
Example 3: Oxidation states in the uses
Give the oxidation state of the metal in [Au(CN)2]−, [Ni(CO)4] and [(Ph3P)3RhCl].
Answer: [Au(CN)2]−: x − 2 = −1, +1. [Ni(CO)4]: CO is neutral, 0. [(Ph3P)3RhCl]: PPh3 is neutral and Cl− is −1, so x − 1 = 0, +1 (reasoned).
Common mistakes to avoid
Common mistake
Mixing up the metals in biological complexes
Chlorophyll contains magnesium, haemoglobin iron and vitamin B12 cobalt.
Common mistake
Taking the EDTA ratio from the charges
EDTA4− and Ca2+ or Mg2+ still react 1 : 1: one hexadentate EDTA wraps one metal ion through two N and four O atoms (NCERT page 121).
Quick revision summary
- Analysis: colour reactions with chelating ligands (EDTA, DMG, α-nitroso-β-naphthol, cupron).
- Hardness of water: titration with NaEDTA; Ca and Mg form stable complexes.
- Metallurgy: gold as [Au(CN)], recovered with zinc; nickel purified through [Ni(CO)].
- Biology: chlorophyll (Mg), haemoglobin (Fe), vitamin B (Co); enzymes such as carbonic anhydrase.
- Industry and medicine: Wilkinson’s catalyst [(PhP)RhCl]; electroplating from [Ag(CN)] and [Au(CN)]; hypo forms [Ag(SO)]; chelate therapy (EDTA for lead); cis-platin.
Formula sheet
- Hypo fixing: AgBr + 2NaSO → Na[Ag(SO)] + NaBr
- Nickel purification: Ni + 4CO → [Ni(CO)] → Ni + 4CO
- EDTA titration: moles of EDTA = moles of (Ca + Mg)
Questions students ask
Which tests in salt analysis use coordination compounds?
Many confirmatory tests of salt analysis form a coordination compound (NCERT Laboratory Manual, Chemistry, Class 12, Unit 7): for chloride, AgCl dissolves in ammonia as [Ag(NH3)2]Cl (page 58), the nitrate test’s brown ring is written in abbreviated form as [Fe(NO)]SO4; the aqueous complex includes water ligands (page 61), and Cu2+ gives the deep-blue ammine complex, conventionally abbreviated [Cu(NH3)4]SO4 (page 74). Fe3+ gives Prussian blue with K4[Fe(CN)6] and blood red [Fe(SCN)]2+ with thiocyanate (page 76), and Ni2+ a brilliant red precipitate with DMG in ammoniacal solution (page 78). All five ions are in the Unit 20 salt analysis lists of JEE Main 2027 and NEET (UG) 2026.
Why does cis-platin fight cancer while trans-platin does not?
cis-Platin, cis-[Pt(NH3)2Cl2], works because inside a cell, where chloride is scarce, its chloride ligands are replaced, by way of water, by nitrogen atoms of DNA, mostly of two neighbouring guanines on one strand; these cross-links interfere with cell division, and a cell that cannot repair them dies (beyond the NCERT text; Wikipedia, “Cisplatin”). In trans-platin the two chlorides point opposite ways, so it cannot link neighbouring guanines like this, and it is thought to react away before reaching the DNA (same source). NCERT names cis-platin among the platinum complexes that inhibit tumours (page 137).
Why is calcium disodium EDTA, not Na2EDTA, used to treat lead poisoning?
Calcium disodium EDTA, CaNa2EDTA, is used against lead poisoning because its calcium-loaded ligand exchanges calcium for more strongly bound lead, avoiding the calcium-stripping action of disodium EDTA: it binds lead in the soft tissues for excretion in urine and bile, while Na2EDTA, which carries no calcium, can cause dangerous hypocalcaemia and has been linked to deaths (beyond the NCERT text; CDC, Morbidity and Mortality Weekly Report, 2006). Its EDTA already holds Ca2+, and lead, which EDTA binds more firmly, takes calcium’s place in the chelate (reasoned). NCERT says only that EDTA is used in the treatment of lead poisoning (page 137); the Na2EDTA of the hardness titration is the reagent, not the drug.
How is the hardness of water expressed in ppm from an EDTA titration?
Hardness from an EDTA titration is usually reported in milligrams per litre as CaCO3, approximately equal to mass ppm for dilute water of density near 1 kg per litre. This is the mass of CaCO3 per litre that would hold the same total moles of Ca2+ and Mg2+, so 1 mmol L of the two ions counts as 100.09 mg L as CaCO (beyond the NCERT text; Wikipedia, “Hard water”). If 100.0 mL of water needs 7.25 mL of 0.0160 M Na2EDTA, the ions amount to 0.0160 × 0.00725 L = 1.16 × 10 mol, or 1.16 × 10 mol L, since EDTA binds them 1 : 1 (reasoned). The hardness is 1.16 × 100.09 ≈ 116 mg L, approximately 116 ppm for dilute water; compare the concentration conventions for parts per million (reasoned).
How does the Mond process purify nickel?
The Mond process, devised by Ludwig Mond in 1890, purifies nickel by passing carbon monoxide over the impure metal at 50–60 °C, which carries the nickel away from the solid impurities as gaseous [Ni(CO)4]; the gas is then heated to 220–250 °C, where it breaks down into pure nickel and CO (beyond the NCERT text; Wikipedia, “Mond process”). [Ni(CO)4] boils at 43 °C, below the working temperature (Wikipedia, “Nickel tetracarbonyl”). Formation reduces the gas-particle count and has a negative reaction entropy. Its exothermic equilibrium therefore favours carbonyl formation at lower temperature and decomposition on heating; the actual direction also depends on CO pressure and composition through ΔG = ΔG° + RT ln Q (Gibbs energy; reasoned).
Are the applications of coordination compounds in the JEE Main 2027 and NEET (UG) 2026 syllabus?
Yes, the applications of coordination compounds are in both syllabuses, but only three are named: Unit 12 of JEE Main 2027 ends with the “importance of coordination compounds (in qualitative analysis, extraction of metals and in biological systems)”, and Unit 12 of NEET (UG) 2026 with “Importance of co-ordination compounds (in qualitative analysis, extraction of metals and in biological systems)”. The rest of NCERT Section 5.7 (pages 136–137), water hardness, catalysts, electroplating, photography and medicine, is named in neither. Unit 20 of both also lists the “Chemical principles involved in the qualitative salt analysis”, whose confirmatory tests often use complexes.
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Discuss briefly, giving an example in each case, the role of coordination compounds in (i) biological systems, (ii) medicinal chemistry, (iii) analytical chemistry and (iv) extraction or metallurgy of metals. (NCERT Exercise 5.27.)
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(i) Haemoglobin, an iron complex, carries oxygen; chlorophyll, a magnesium complex, is the photosynthetic pigment. (ii) EDTA is used to treat lead poisoning (NCERT page 137; the drug is edetate calcium disodium, beyond the NCERT text, as above); cis-platin inhibits tumour growth. (iii) DMG and EDTA detect and estimate metal ions; hardness is found by titration with Na2EDTA. (iv) Gold is leached as [Au(CN)2]− and recovered with zinc; nickel is purified through [Ni(CO)4] (NCERT pages 136–137).
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Q2. Why is silver plated from a solution of [Ag(CN)2]− rather than of Ag+ ions?
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Articles plate much more smoothly and evenly from the complex than from a solution of the simple metal ions (NCERT page 137). The complex is stable, so the solution holds very little free Ag+ at any moment, which helps control deposition and improve uniformity. The rate also depends on current density, potential and transport, not complex stability alone (reasoned).