All subjects · Chemistry · WCH15 (Unit 5)

Bidentate and hexadentate ligands, chelates and haemoglobin

Some ligands bond to a metal ion through two or more lone pairs at once. These complexes are much more stable, and the reason is entropy. Haemoglobin is a natural example.

Polydentate ligands (spec 17.16)

Bidentate ligands form two dative bonds each: 1,2-diaminoethane (en), H2NCH2CH2NH2, bonds through both N atoms; ethanedioate, C2O4²-, through two O atoms. Three en ligands give an octahedral complex, [Ni(en)3]2+, coordination number 6.
Hexadentate ligands form six dative bonds: EDTA4- wraps around a metal ion using two N and four O atoms, giving a 1 : 1 octahedral complex.

The chelate effect (spec 17.25)

Replacing monodentate ligands with polydentate ones increases the number of particles in solution:

[Cu(H2O)6]2+ + 3en -> [Cu(en)3]2+ + 6H2O (4 particles become 7)

[Cu(H2O)6]2+ + EDTA4- -> [Cu(EDTA)]2- + 6H2O (2 particles become 7)

The bonds broken and made are similar (Cu-O for Cu-N), so ΔH is small, but ΔSsystem is large and positive. So ΔStotal is positive and the polydentate complex is more stable. EDTA is used to remove metal ions, for example in water softening and treating metal poisoning.

Haemoglobin (spec 17.17)

Haemoglobin contains iron(II) held by a polydentate ring ligand (haem) and a protein. Fe2+ is six-coordinate; one position binds O2 in the lungs and releases it in tissues. Carbon monoxide is a better ligand: it replaces O2 by ligand exchange and binds much more strongly, so the haemoglobin can no longer carry oxygen. This is why CO is toxic.

Worked example

Explain why [Cu(EDTA)]2- forms when EDTA4- is added to [Cu(H2O)6]2+, although ΔH is close to zero.

2 particles on the left become 7 on the right.
ΔSsystem is large and positive; ΔSsurroundings is close to zero.
So ΔStotal is positive.

Answer: Entropy drives the reaction: the chelate complex is more stable

Key idea: Bidentate (en, ethanedioate): 2 bonds; hexadentate (EDTA4-): 6 bonds. Swapping monodentate for polydentate ligands increases the number of particles, so ΔSsystem > 0 and the complex is more stable. CO replaces O2 on Fe2+ in haemoglobin.

Check yourself

How many dative bonds does one 1,2-diaminoethane ligand form with a metal ion?
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2. It has a lone pair on each of its two nitrogen atoms.
What is the coordination number of nickel in [Ni(en)3]2+?
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6. 3 bidentate ligands x 2 bonds = 6.
How many dative bonds does EDTA4- form with a metal ion?
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6. EDTA4- is hexadentate.
What is the sign of ΔSsystem for [Cu(H2O)6]2+ + EDTA4- -> [Cu(EDTA)]2- + 6H2O?
- Negative, because a large complex forms
- Zero, because the same bonds form
- Positive, because 2 particles become 7
- Negative, because water is released
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Positive, because 2 particles become 7. More particles means more disorder.
Why is ΔH close to zero when en replaces water ligands around Cu2+?
- Similar numbers and types of bonds to the metal are broken and made
- No bonds are broken
- en ligands form ionic bonds
- Water molecules have no lone pairs
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Similar numbers and types of bonds to the metal are broken and made. Six Cu-O dative bonds are replaced by six similar Cu-N dative bonds.
Why is carbon monoxide toxic?
- It oxidises iron(II) to iron(III)
- It removes iron from haemoglobin
- It replaces oxygen as a ligand on the iron(II) in haemoglobin and binds more strongly
- It reacts with water in the blood
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It replaces oxygen as a ligand on the iron(II) in haemoglobin and binds more strongly. Ligand exchange by CO blocks oxygen transport.
What is the oxidation state of iron in haemoglobin?
- +2
- +3
- 0
- +6
Show answer
+2. Haemoglobin contains iron(II).

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Last updated 7 November 2026. Spotted a mistake? Tell us.