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Learn9701Paper 4 · A Level Structured QuestionsChemistry of transition elementsComplex ions, ligands and colour
Chemistry of transition elements · Subtopic 2 of 3

Complex Ions, Ligands and Colour

A complex ion is a central metal ion with ligands bonded by dative (coordinate) bonds. Coordination number = number of dative bonds: 6 → octahedral ([Cu(H₂O)₆]²⁺); 4 → tetrahedral ([CuCl₄]²⁻, with large Cl⁻) or square planar (cisplatin, Pt); 2 → linear ([Ag(NH₃)₂]⁺). Ligands may be monodentate (H₂O, NH₃, Cl⁻), bidentate (en, C₂O₄²⁻) or polydentate (EDTA⁴⁻, 6 bonds). Colour: ligands split the d orbitals; an electron absorbs visible light of energy ΔE=hν\Delta E=h\nu to jump up; the complementary colour is seen. Changing ligand, coordination number or oxidation state changes ΔE and so the colour. Ligand exchange is favoured by larger KstabK_\text{stab} and by the chelate effect (entropy increase). Complexes show cis/trans and optical isomerism.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 1≈ 55 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define ligand, complex and coordination number.
  • Predict shapes of complexes.
  • Explain colour using d-orbital splitting.
  • Describe ligand exchange, stability constants and stereoisomerism.
Pause and think

Copper sulfate solution is blue, but anhydrous copper sulfate is white. Why?

Section 2 of 14

Core idea

Start hereBlue, yellow, deep blue

The same copper ion can be pale blue in water, yellow-green in concentrated HCl and deep blue in ammonia. Only the ligands change.

The big idea

Ligands donate lone pairs to a central ion and split its d orbitals. The size of the split decides which light is absorbed, and so the colour we see.

1. Shapes

Coordination number and shape

Count dative bonds, not ligands.

2. Colour

Why [Cu(H₂O)₆]²⁺ looks blue
  1. d orbitals split by ligands\text{d orbitals split by ligands}
    Ligand lone pairs repel some d orbitals more than others.
  2. ΔE=hν\Delta E=h\nu
    An electron absorbs a photon of matching energy and moves up.
  3. absorbs red–orange\text{absorbs red–orange}
    ΔE for water ligands matches roughly 800 nm.
  4. transmits blue\text{transmits blue}
    We see the complementary colour.
Worked exampleChelate effect

Why is [Cu(H2O)6]2++3en→[Cu(en)3]2++6H2O\mathrm{[Cu(H_2O)_6]^{2+}+3en\rightarrow[Cu(en)_3]^{2+}+6H_2O} so favourable?

Goal
Explain using ΔS.
Strategy
Count particles.
  1. 1+3=4 particles→1+6=71+3=4\ \text{particles}\rightarrow1+6=7
    More particles on the right.
  2. ΔS>0, ΔH≈0⇒ΔG<0\Delta S>0,\ \Delta H\approx0\Rightarrow\Delta G<0
    Similar Cu–N and Cu–O bond strengths.
AnswerEntropy increases, so ΔG is negative.
✓ CheckConsistent with large Kstab ✓.

Summary

  • Dative bonds from ligands.
  • Shapes from coordination number.
  • Colour: complementary to absorbed light.
  • Chelate effect; cis/trans and optical isomers.
Key words
Ligand
Species donating a lone pair to a central metal ion.
Bidentate
Ligand forming two dative bonds.
Chelate effect
Multidentate ligands replace monodentate ones due to entropy increase.
HistoryCisplatin by accident. In 1965 Barnett Rosenberg noticed bacteria stopped dividing near platinum electrodes. The cause was cisplatin, now one of the most widely used cancer drugs.
Fun factHaemoglobin is a complex. Each haem group holds Fe²⁺ with four N atoms from a ring plus a protein histidine; O₂ binds as the sixth ligand. CO binds more strongly, which is why it's poisonous.
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