Substitution Reactions (A2 Only)
Chemistry: AQA A Level, topic 4.2.2
Substitution Reactions
The ligands of complex ions can be exchanged for other ligands. This is called a substitution reaction.
Common ligands
- Some common ligands are water, NH3, and Cl-.
- NH3 and water are of a similar size and have no charge.
- Chloride ions are larger and have a negative charge.
Substitution reactions
- Ligands can be exchanged.
- A good example is the interconversion of [Cu(H2O)6]2+ and [Cu(NH3)6]2+
- Substitution could also be incomplete.
- E.g. [Cu(NH3)6]2+ to [Cu(NH3)4(H2O)2]2+
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Coordination number
- The coordination number can change in a ligand substitution reaction.
- This usually happens when substituting ligands of a different size.
- E.g. [Co(H2O)6]2+ + 4Cl- → [CoCl4]2-
- The coordination number does not change when substituting water for ammonia because they are of a similar size.
The Chelate Effect
Some ligands can form multiple dative bonds with transition metal ions. This leads to some important chemical reactions.
Denticity
- Denticity describes the number of dative bonds that can be formed with a transition metal by one ligand:
- A ligand that can form two bonds is called bidentate.
- A ligand that can form one bond is called monodentate.
- A ligand that can form multiple bonds is called multidentate.
- Some common bidentate and multidentate ligands are on the next slide.
Multidentate ligands
- Binding sites are labelled by red circles.
The chelate effect
- Multidentate ligands will almost always replace monodentate ligands at a metal centre. This is called the chelate effect.
- The chelate effect is largely an entropic effect.
- When you substitute a multidentate ligand for a monodentate ligand, you increase the number of moles of molecules in the products of the reaction.
- This leads to a large increase in entropy and this brings the Gibbs free energy below zero for the change.
Haem protein
- The haem protein is found in red blood cells.
- It has an iron(II) ion in the centre of a multidentate ligand.
- In the lungs, oxygen binds to it as a ligand and can then be transported around the body by the protein.
- In the body, the oxygen concentration is lower than in the lungs. So the dissociation of the oxygen is favoured.
Carbon monoxide
- When you inhale carbon monoxide, the CO can form a bond with the iron instead of oxygen.
- CO forms an extremely strong bond with the iron. This will not break.
- This means that the haem protein can no longer carry oxygen around the body.
- This can starve the body of oxygen and cause death if enough is inhaled.
- This is a ligand exchange reaction.
Test yourself on Substitution Reactions (A2 Only)
Ligands can replace other ligands in complexes via what type of reaction?
Why does the coordination number NOT change when exchanging ammonia and water?