Substitution reactions (A-level only)

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.

Students should be able to:

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

AT d and k