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Coordination number questions go wrong for one reason more than any other: students count ligands instead of donor atoms. Fix that and denticity, chelation and ambidentate ligands fall into place. This post defines each term, works through counting examples and explains why chelating ligands bind more tightly.
Where does this sit in the syllabus?
Section 5.2 of the rationalised NCERT Class 12 chapter on Coordination Compounds (Reprint 2026-27) defines ligands, denticity, chelates and ambidentate ligands, and Section 5.4.3 covers linkage isomerism. The JEE (Main) 2026 syllabus names "ligands, coordination number, denticity, chelation" in Unit 12. Check the syllabus for your exam year.
What is denticity?
NCERT's definitions:
Unidentate: binds through one donor atom, such as Cl⁻, H₂O or NH₃.
Didentate: binds through two donor atoms, such as ethane-1,2-diamine (en, H₂NCH₂CH₂NH₂) or oxalate (C₂O₄²⁻).
Polydentate: several donor atoms, such as N(CH₂CH₂NH₂)₃. EDTA⁴⁻ is hexadentate: it binds through two nitrogen and four oxygen atoms.
The number of donor groups a ligand uses is its denticity. When a di- or polydentate ligand uses two or more donor atoms to bind the same metal ion, it is a chelate ligand. OpenStax Chemistry 2e explains the name: chelate comes from the Greek for claw, and a chelating ligand holds the metal "rather like a crab's claw would hold a marble".
What makes a ligand ambidentate?
An ambidentate ligand has two different donor atoms but uses only one at a time. NCERT's examples are NO₂⁻, which can bind through nitrogen or oxygen, and SCN⁻, which can bind through sulphur or nitrogen. This gives linkage isomers. NCERT describes Jørgensen's two forms of [Co(NH₃)₅(NO₂)]Cl₂: a red form bound through oxygen (–ONO) and a yellow form bound through nitrogen (–NO₂).
Ambidentate is not the same as didentate. Nitrite has two possible donor atoms, but in these complexes it uses one, so it is still unidentate.
Worked examples: coordination number and oxidation state
[Co(en)₃]³⁺. Three didentate en ligands give 3 × 2 = 6 donor atoms. Coordination number 6. en is neutral, so Co is +3. NCERT uses this example.
[Fe(C₂O₄)₃]³⁻. Three didentate oxalates: coordination number 6. Charges: x + 3(−2) = −3, so x = +3.
[Ca(EDTA)]²⁻. One hexadentate EDTA⁴⁻: coordination number 6. Charges: x + (−4) = −2, so Ca is +2.
[Co(NH₃)₅(NO₂)]Cl₂. Two Cl⁻ outside the bracket mean the complex ion is [Co(NH₃)₅(NO₂)]²⁺. Five NH₃ and one NO₂⁻ give coordination number 6. Charges: x + 0 + (−1) = +2, so Co is +3.
[PtCl₆]²⁻ and [Ni(NH₃)₄]²⁺. Coordination numbers 6 and 4, as NCERT states. Pt is +4 and Ni is +2.
Why are chelate complexes more stable?
NCERT states that chelate complexes tend to be more stable than similar complexes containing unidentate ligands. This is the chelate effect. Bowman (2006) describes a classroom demonstration in which colour changes show didentate ligands displacing unidentate ones from copper(II) and nickel(II), and discusses the roles of enthalpy and entropy. Frausto da Silva (1983) points out that the statement is widely accepted but its exact meaning is often misunderstood, so be precise about what "more stable" compares.
A particle count shows the entropy part of the argument. Consider replacing six NH₃ with three en on Ni²⁺:
[Ni(NH₃)₆]²⁺ + 3 en → [Ni(en)₃]²⁺ + 6 NH₃
Four particles on the left become seven on the right. The bonds broken and made are similar, metal to nitrogen each time, but the number of free particles rises, which favours the products. A common way to picture it: once one end of en is bound, the other end is held close to the metal, so it rebinds easily if it lets go. Each en with the metal forms a five-membered ring: M, N, C, C, N.
Where are chelates used?
NCERT lists several uses. Water hardness is estimated by titration with Na₂EDTA, because Ca²⁺ and Mg²⁺ form stable EDTA complexes. EDTA is used to treat lead poisoning, and D-penicillamine and desferrioxime B remove excess copper and iron. Reagents such as EDTA and dimethylglyoxime (DMG) are used to detect and estimate metal ions.
Which mistakes cost marks?
Counting ligands, not donor atoms, for coordination number.
Calling an ambidentate ligand didentate.
Forgetting counter-ions when finding the charge on the complex ion.
Treating EDTA as having charge −2. In NCERT's ligand list it is EDTA⁴⁻.
A practice routine
Day 1: List ten ligands with their charge, denticity and donor atoms.
Day 2: Find coordination number and oxidation state for ten complexes, including three with chelates.
Day 3: Draw both linkage isomers of a nitrite and a thiocyanate complex.
Day 4: Explain the chelate effect in four lines, using the particle count.
Next, use these counts in the hybridisation method from our post on valence bond theory for complexes. If a term like denticity keeps slipping, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson.
Tonight, find the coordination number and oxidation state in K₃[Cr(C₂O₄)₃], and check that you get 6 and +3.
