Substitution Reactions
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Ligand Substitution or Exchange Reaction What does this usually cause?
Where one ligand can be swapped for another. A colour change
Effect on the complex ion if the ligands are of similar size and same charge
The co-ordination number of the complex ion doesn’t change, and neither does the shape
Colour and shape change in cobalt from H2O to NH3 ligands What happens if the ammonia isn’t in excess? Give formula?
Octahedral pink in H2O to Octahedral deep blue in NH3. A blue precipitate forms. Cu (OH)2 (H2O)4
Effect of complex ion if the ligands are different sizes
There’s a change in: • Co ordination number • Change of shape
Colour and shape change in cobalt from H2O to Cl ligands
Octahedral pink in H2O to Tetrahedral blue in Cl
Colour and shape change in copper from H2O to Cl ligands
Octahedral pale blue in H2O to Tetrahedral yellow in Cl
Colour and shape change in copper from H2O to Cl ligands
Octahedral purple in H2O to Tetrahedral yellow
When is the change in ligands easy to reverse?
If the new ligands form stronger bonds with the central metal ion than the old ligands did
Multidentate ligands and reversibility of substitution reactions
Multidentate ligands form more stable complexes than monodentate ligands. So the change in ligands is hard to reverse
Enthalpy change for a ligand substitution reaction and describe
When ligand substitution/exchange reactions occur, co-ordinate bonds are broken and formed. The strength of the bonds being broken is often very similar to the strength of the the new bonds being made. So the enthalpy change for a ligand exchange reaction is usually very small
Chelate Effect and Entropy
Explains increase in stability. When monodentate ligands are substituted with bidentate or multidentate ligands, the number of particles in solution increases. The more particles, the greater the entropy. Reactions that result in increases in entropy are more likely to occur
Why is it difficult to reverse certain substitution reactions?
Because reversing them would cause a decrease in entropy