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Substitution Reactions



Transition Metals

AQA Content

Explain the chelate effect, in terms of the balance between the entropy and enthalpy change in these reactions.

Specification Notes

H2O, NH3 and Cl− can act as monodentate ligands.
The ligands NH3 and H2O are similar in size and are uncharged.
Exchange of the ligands NH3 and H2O occurs without change of co-ordination number (eg Co2+ and Cu2+).
Substitution may be incomplete (eg the formation of [Cu(NH3)4(H2O)2]2+).
The Cl− ligand is larger than the uncharged ligands NH3 and H2O
Exchange of the ligand H2O by Cl– can involve a change of co-ordination number (eg Co2+, Cu2+ and Fe3+).
Ligands can be bidentate (eg H2NCH2CH2NH2 and C2O42–).
Ligands can be multidentate (eg EDTA4–).
Haem is an iron(II) complex with a multidentate ligand.
Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.
Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.
Bidentate and multidentate ligands replace monodentate ligands from complexes. This is called the chelate effect.



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