5.3 - Transition elements

Question 1

Transition metal ions can bond to ligands to form complex ions with different shapes. Explain what is meant by the terms ligand, coordination number and ligand substitution. (3)

Answer 1

• Ligand: Donates a lone pair of electrons to metal ion in order to form dative covalent (coordinate) bond with metal ion
• Coordination number: Number of coordinate bonds to metal ion
• Ligand substitution: One ligand replacing another

Question 2

Red blood cells contain haemoglobin.
Explain using ligand substitutions:
- how haemoglobin transports oxygen around the body
- why carbon monoxide is toxic. (3)

Answer 2

Coordinate bond mark:
- O2 (coordinately or datively) bonds with Fe2+/Fe(II)/Fe/Iron
Ligand substitution mark:
- (When required) O2 is replaced by H2O / CO2
Ligand strength mark:
- CO forms strong(er) bonds (than O2) / CO bonds irreversibly