Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.5.2Inorganic Chemistry

Substitution Reactions

5 min read 5 parts Lesson 19 of 27

0%read

Section 1 of 5Ligands and Substitution

2.5.2 Substitution Reactions
Ligands and Substitution
Ligand
  • A ligand has a lone pair of electrons that forms a coordinate bond to the central metal ion.
Coordination number
  • The number of coordinate bonds from ligands to the central ion, commonly 4 or 6.
Substitution
  • One ligand is replaced by another when a new ligand is added in a high enough concentration.
Similar size
  • Water and ammonia are similar in size and charge, so substitution does not change the coordination number.

Section 2 of 5Examples

Copper(II) and ammonia
  • [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O;
    • pale blue changes to deep blue, and substitution is incomplete.
Cobalt(II) and ammonia
  • [Co(H₂O)₆]²⁺ + 6NH₃ ⇌ [Co(NH₃)₆]²⁺ + 6H₂O; the pink solution changes to straw-coloured.
Copper(II) and chloride
  • [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O; blue changes to yellow-green and coordination number falls to 4.
Cobalt(II) and chloride
  • [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O; pink changes to blue.

Section 3 of 5Why the Coordination Number Changes

1
Chloride ions are much larger than water or ammonia molecules.
2
Only four chloride ions can fit around the central metal ion without crowding.
3
Six water ligands are therefore replaced by four chloride ligands, so the complex changes from octahedral to tetrahedral.
4
The reactions are reversible, so adding water to a chloro complex reverses the change.
5
Dilution with water pushes the equilibrium back towards the aqua complex.

Section 4 of 5Bidentate Ligands and the Chelate Effect

Bidentate Ligands and the Chelate Effect
Bidentate
  • Ethane-1,2-diamine (en) has two donor nitrogen atoms and forms two coordinate bonds per molecule.
Multidentate
  • EDTA⁴⁻ is hexadentate and forms six bonds to one metal ion.
Example
  • [Ni(H₂O)₆]²⁺ + 3en ⇌ [Ni(en)₃]²⁺ + 6H₂O; four particles on the left become seven on the right.
Chelate effect
  • The rise in particle number gives a positive entropy change, so the complex with the multidentate ligand is more stable.

Section 5 of 5Exam Checklist & Tips

2.5.2 – Key Points for the Exam

  • Ligand substitution A reaction in which one or more ligands in a complex are replaced by others.
  • Coordination number The number of coordinate bonds to the central metal ion.
  • Bidentate ligand A ligand that forms two coordinate bonds to the metal ion.
  • Chelate effect The extra stability of complexes with multidentate ligands because of the entropy increase.

Common Mistakes

  • Saying that ammonia and water substitution changes the coordination number.
  • Writing the product of copper(II) with concentrated chloride as an octahedral ion.
  • Showing the ligand's lone pair as coming from the metal.
  • Forgetting that ligand substitution is usually an equilibrium.
\n