5.6.4Physical Chemistry and Transition Elements
Ligand Substitution Reactions — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 4 parts Lesson 37 of 40
Part 1 of 4Substitution Reactions
In a ligand substitution one ligand in a complex is replaced by another.
Copper(II) with ammonia
- [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O, changing from pale blue to deep blue.
Copper(II) with chloride
- [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O, changing from blue to yellow in concentrated hydrochloric acid.
Cobalt(II) with chloride
- [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O, changing from pink to blue.
Iron(III) with thiocyanate
- [Fe(H₂O)₆]³⁺ + SCN⁻ → [Fe(SCN)(H₂O)₅]²⁺ + H₂O, a blood-red colour used to detect Fe³⁺.
Part 2 of 4Change of Coordination Number
The size of the incoming ligand decides whether the coordination number changes.
1
Water and ammonia are similar in size and uncharged, so replacing water by ammonia does not change the coordination number.
2
The chloride ion is larger and carries a negative charge, so only four fit around the metal ion.
3
Six H₂O ligands are replaced by four Cl⁻ ligands, so the coordination number falls from 6 to 4 and the shape becomes tetrahedral.
4
The reactions of chloride are reversible; adding water to the yellow [CuCl₄]²⁻ turns the solution blue-green and then blue again.
5
The colour changes occur because new ligands alter the splitting of the d orbitals and thus the wavelengths of light absorbed.
Part 3 of 4Chelate Effect and Stability
Chelate Effect and Stability
Equation
- [Cu(H₂O)₆]²⁺ + 2en → [Cu(en)₂(H₂O)₂]²⁺ + 4H₂O, where bidentate en replaces four water ligands.
Particle count
- Three particles on the left produce five on the right, so the entropy of the system increases.
Enthalpy
- Similar numbers of bonds of similar strength are formed and broken, so ΔH is close to zero.
Consequence
- ΔG = ΔH − TΔS is negative because of the positive ΔS, so the chelate complex is more stable than the aqua complex.
Part 4 of 4Exam Checklist & Tips
5.6.4 – Key Points for the Exam
- Ligand substitution A reaction in which one or more ligands in a complex are replaced by others.
- Chelate effect The extra stability of complexes with bidentate or multidentate ligands, caused by an increase in entropy.
- Aqua complex A complex in which water molecules are the ligands.
- Coordination number change A change in the number of ligands bonded, which may alter the shape.
Common Mistakes
- Giving the deep blue ammonia complex of copper(II) the formula [Cu(NH₃)₆]²⁺ rather than [Cu(NH₃)₄(H₂O)₂]²⁺.
- Stating that a chelate effect is due to a more exothermic enthalpy change rather than a more positive entropy change.
- Forgetting to state that the chloride complexes are tetrahedral, not octahedral.
- Writing the charge of [CuCl₄]²⁻ as 2+ because of the charge on the copper ion.
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