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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

5.3.1 Transition elements

Ligand substitutionLigands and complex ionsPrecipitation reactionsPropertiesRedox reactions

Ligand Substitution

Specification Reference Topic 5.3.1 (a)–(l)

Quick Notes

  • Ligand Substitution with ammonia (NH3):
    • [Cr(H2O)6]3+ → [Cr(NH3)6]3+
      (pale blue → deep blue solution)
    • [Cr(H2O)6]3+ → [Cr(NH3)6]3+
      (pale purple → purple solution)
  • Ligand Substitution With chloride ions (Cl−):
    • [Cu(H2O)6]2+ + 4Cl− → [CuCl4]2−
      (pale blue) → (yellow solution)
  • Iron in Haemoglobin:
    • Fe2+ binds O2 for transport in blood.
    • CO binds more strongly than O2, preventing oxygen transport (carbon monoxide poisoning).

Full Notes

Ligands in a complex ion can sometimes be substituted for different ligands in what are called ligand substitution or ligand exchange reactions.

Substitution of H2O by NH3

Only six H2O or NH3 ligands can co-ordinately bond to the metal ion. The coordination number remains 6.

For Example [Cr(H2O)6]3+ + 6NH3 → [Cr(NH3)6]3+ + 6H2O

Complete ligand substitution forms a stable octahedral complex.

Substitution of H2O by Cl−

Cl− ligands are larger, so only four can fit around the central metal ion. Coordination number changes 6 → 4, octahedral → tetrahedral.

For Example [Cu(H2O)6]2+ + 4Cl− ⇌ [CuCl4]2− + 6H2O

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Matt’s exam tip

Don’t confuse precipitation with ligand substitution. For example, Cu2+ + 2OH− → Cu(OH)2(s) is deprotonation, not ligand exchange.

Ligand Substitution Reactions

These are the reactions you need to be able to remember - a revision sheet can be downloaded here that also includes hydroxide precipitation reactions.

Copper(II) and Ammonia

OCR (A) A-Level Chemistry ligand substitution with copper(II) and ammonia.

Start with [Cu(H2O)6]2+ (pale blue).

Add NH3 (few drops): Cu(OH)2(s) precipitate forms.

Add excess NH3:

[Cu(H2O)6]2+ + 4NH3 → [Cu(NH3)4(H2O)2]2+ + 4H2O

Deep blue solution forms (octahedral).

Copper(II) and Chloride Ions

OCR (A) A-Level Chemistry ligand substitution with copper(II) and chloride ions.

[Cu(H2O)6]2+ (pale blue) + 4Cl− ⇌ [CuCl4]2− + 6H2O (yellow-green solution). Tetrahedral complex. Reversible with water.

Chromium(III) and Ammonia

OCR (A) A-Level Chemistry ligand substitution with chromium(III) and ammonia.

[Cr(H2O)6]3+ (violet) + 6NH3 → [Cr(NH3)6]3+ + 6H2O (purple). Complete substitution, stable octahedral complex.

Haemoglobin and Oxygen Transport

OCR (A) A-Level Chemistry haemoglobin with oxygen ligand binding to Fe2+.

Fe2+ in haemoglobin binds O2 via a coordinate bond to transport oxygen in blood.

OCR (A) A-Level Chemistry haemoglobin ligand substitution with carbon monoxide vs oxygen.

CO binds more strongly than O2, blocking oxygen transport → carbon monoxide poisoning.

Summary