Chelation — Why ring-forming ligands grip metals tighter
A polydentate ligand that forms a ring structure with the central metal ion produces a chelate. The word comes from the Greek chela (claw) — the ligand wraps around the metal through two or more donor atoms within the same molecule. Ethylenediamine (en), EDTA, and oxalate (C₂O₄²⁻) are standard examples from NCERT Class 12 Chemistry Chapter 5.
What makes chelation thermodynamically favourable? The chelate effect: replacing two monodentate ligands with one bidentate ligand increases the number of free species in solution (entropy rises). For the reaction [Cu(H₂O)₄]²⁺ + 2 en → [Cu(en)₂]²⁺ + 4 H₂O, you start with 3 species and end with 5. This positive ΔS drives the equilibrium forward even when ΔH is comparable to the monodentate case.
Chelate ring size matters. Five-membered chelate rings (formed by en, glycinate) are the most stable. Four-membered rings (formed by carbonate binding through both oxygens) are strained. Six-membered rings (acetylacetonate) are stable but less common at NEET level.
Denticity vs. coordination number — a frequent confusion. Denticity is how many donor atoms one ligand provides. Coordination number is the total number of donor atoms bonded to the metal from all ligands. EDTA is hexadentate (6 donor atoms from one molecule), so one EDTA occupies all 6 coordination sites of an octahedral metal ion.
NEET relevance. Questions test whether you can identify chelating ligands, count denticity correctly, explain why chelates are more stable, and distinguish chelation from simple coordination. The definition of chelate itself (NCERT Class 12 Chemistry Chapter 5, page 4) is directly evaluable.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following ligands can act as a chelating agent?
The term 'chelate' is derived from the Greek word *chela*, which means:
The denticity of EDTA as a ligand is:
In the complex [Co(en)₃]³⁺, the coordination number of cobalt is:
Why are chelate complexes generally more stable than analogous complexes with monodentate ligands?
Which of the following chelate ring sizes is generally the most stable?
Glycinate ion (H₂N–CH₂–COO⁻) acts as a bidentate ligand. Through which donor atoms does it coordinate to a metal ion?
Consider the reaction: [Ni(H₂O)₆]²⁺ + 3 en → [Ni(en)₃]²⁺ + 6 H₂O. How does the total number of species change from reactants to products, and what is the thermodynamic consequence?
Given
- Reaction: [Cu(H₂O)₄]²⁺ + 2 en → [Cu(en)₂]²⁺ + 4 H₂O - Individual Cu–N bond energies (in en complex) are comparable to Cu–O bond energies (in aqua complex).
Required
Explain why the chelate complex is more stable. Count independent species on each side and identify the thermodynamic driving force.
Concept
The chelate effect: when polydentate ligands replace monodentate ligands, the total number of free species in solution increases, causing a positive entropy change (ΔS > 0). Since ΔG = ΔH − TΔS and ΔH is approximately similar, the positive ΔS makes ΔG more negative, favouring the chelate.
Formula / Principle
ΔG = ΔH − TΔS If ΔH(chelate) ≈ ΔH(monodentate), the sign of ΔG is controlled by the −TΔS term. Positive ΔS → negative contribution → more negative ΔG → more stable product.
Substitution (species counting)
Reactant side: [Cu(H₂O)₄]²⁺ (1) + en (2) = **3 independent species** Product side: [Cu(en)₂]²⁺ (1) + H₂O (4) = **5 independent species** Change: 3 → 5 species. Net increase = +2 free particles.
Calculation / Reasoning
More free particles in solution → greater translational entropy → ΔS > 0. Since ΔH ≈ 0 (comparable bond strengths), ΔG ≈ −TΔS < 0. The equilibrium lies strongly toward the chelate product.
Final answer
The chelate complex [Cu(en)₂]²⁺ is more stable because the replacement of 4 monodentate H₂O ligands by 2 bidentate en ligands increases the number of free species from 3 to 5, giving ΔS > 0 and thus a more negative ΔG. This entropy-driven stability advantage is the chelate effect.
Common trap
Assuming chelate stability comes from stronger individual bonds. It does not — the Cu–N bond in [Cu(en)₂]²⁺ is not dramatically stronger than the Cu–O bond in [Cu(H₂O)₄]²⁺. The driving force is entropy (more free particles), not enthalpy.
Similar NEET-style question
"Explain why [Ni(en)₃]²⁺ has a much larger formation constant than [Ni(NH₃)₆]²⁺, even though both involve Ni–N bonds." (Answer: same chelate effect reasoning — 4 → 7 species for en substitution vs. 7 → 7 for NH₃, so the en pathway gains entropy.) ---
Ligand: ion or neutral molecule donating electron pair to metal (Lewis base). Monodentate (1 site, e.g. NH₃, Cl⁻). Bidentate (2 sites, e.g. en, ox²⁻). Polydentate (chelate, e.g. EDTA⁴⁻ has 6 sites). Coordination number = number of donor atoms attached.
-- NCERT Class 12 Chemistry, Ch. 5, p. 4Tetrahedral splitting is smaller than octahedral due to fewer/farther ligands.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Delta_o | octahedral splitting | J or eV |
| Delta_t | tetrahedral splitting | J or eV |
Same spin-only formula but n depends on high-spin/low-spin from CFT.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student defaults to one spin state. Strong-field ligand (CN⁻, CO, NH₃ for some) → low-spin (Δ > P, electrons pair). Weak-field (F⁻, H₂O, Cl⁻) → high-spin.
Coordination compound with given ligand asking for magnetic moment, color, or spin state.
Memorise spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO. NH₃, CN⁻, CO usually strong-field. F⁻, H₂O, Cl⁻ usually weak-field.
Root cause: concept gap
Strong-field ligand (CN⁻, CO, NH₃ when applicable): Δ > P → low-spin. Weak-field (F⁻, Cl⁻, H₂O): Δ < P → high-spin. Use spectrochemical series.
11 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Homoleptic complex from the following complexes is
Identify the incorrect statement from the following.
The IUPAC name of the complex- [Ag(H O) ][Ag(CN) ] is: 2 2 2
Ethylene diaminetetraacetate (EDTA) ion is :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
forgets weak vs strong ligand
Default low-spin always
wrong order ligands
Uses non-alphabetical order
counts counter ions as ligands
Treats all attached species as ligands
Test yourself on this topic with real past-paper questions:
Practice this topic →Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.