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5.2.5Unit 5. Transition Metals and Organic Nitrogen Chemistry

Ligand Exchange — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 17 of 43 Unit quiz

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Section 1 of 6What Happens

5.2.5 Ligand Exchange
Definition
  • In ligand exchange one or more ligands in a complex are replaced by other ligands, and the metal ion stays the same.
Similar size
  • Water and ammonia are of similar size, so exchange between them keeps the coordination number at six.
Different size
  • Chloride ions are larger, so only four fit around the metal and the coordination number falls from six to four.
Equilibrium
  • Many exchanges are reversible, and the position depends on the concentration of the new ligand.

Section 2 of 6Copper(II) Examples

1
[Cu(H₂O)₆]²⁺ is pale blue in aqueous solution.
2
With a little ammonia a pale blue precipitate of Cu(OH)₂ forms, as ammonia acts first as a base.
3
With excess ammonia the precipitate dissolves: [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O, giving a deep blue solution.
4
With concentrated hydrochloric acid: [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O, giving a yellow-green solution.
5
Adding water to the chloro complex shifts the equilibrium back and the solution returns to blue.

Section 3 of 6Cobalt(II) Example

Cobalt(II) Example
Aqua ion
  • The hexaaqua ion [Co(H₂O)₆]²⁺ is pink in solution and has an octahedral shape.
Chloro complex
  • Adding concentrated HCl forms [CoCl₄]²⁻, which is blue and tetrahedral: [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O.
Reversal
  • Dilution or cooling pushes the equilibrium back, so the pink colour returns.

Section 4 of 6Why Exchange Happens

Why Exchange Happens
Thermodynamics
  • The new complex is formed when it is more stable than the original, usually judged by ΔG or by the stability constant.
Entropy
  • Replacing six water ligands with four chloride ligands releases more free particles, increasing entropy.
Fixed oxidation state
  • The oxidation number of the metal is unchanged, which distinguishes ligand exchange from redox.

Section 5 of 6Worked Example

Question Balance the charge and atoms for the formation of tetrachlorocobaltate(II) from the hexaaqua ion and chloride ions.

  1. Reactants[Co(H₂O)₆]²⁺ (charge +2) and 4Cl⁻ (charge -4), total -2.
  2. Product[CoCl₄]²⁻ (charge -2), so the six water ligands are released.
  3. Check atoms6 H₂O on both sides, 4 Cl and 1 Co.

Answer [Co(H₂O)₆]²⁺ + 4Cl⁻ → [CoCl₄]²⁻ + 6H₂O

Section 6 of 6Exam Checklist & Tips

5.2.5 – Key Points for the Exam

  • Ligand exchange Replacement of one or more ligands by others in a complex.
  • Tetrahedral Four ligands arranged around the metal at about 109.5°.
  • Octahedral Six ligands arranged around the metal at 90°.
  • Stability constant Equilibrium constant for formation of a complex from its ions.

Common Mistakes

  • Writing [CuCl₄]²⁻ with a charge of 2+.
  • Suggesting that the oxidation state of copper changes in ligand exchange.
  • Giving the pale blue precipitate as the final product with excess ammonia.
  • Forgetting that the coordination number changes with large ligands.

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