Haemoglobin & Ligand Exchange (Edexcel IAL Chemistry)

Spec point — 17.17

Haemoglobin & Ligand Exchange

• Haemoglobin is one of nature's complexes using a transition metal ion

• The haem molecule is a complex with iron(II) at its centre

• The haemoglobin complex contains a multidentate ligand made up of four haem groups

  • These consist of mostly carbon and hydrogen atoms
  • Each haem group has a nitrogen atom forming a dative covalent bond to the Fe²⁺ ion in a square planar complex

• There is a fifth dative bond from the protein (globin) to the Fe²⁺ ion

• Oxygen atoms form a dative covalent bond with the iron(II) which enables oxygen molecules to be transported around the body in the blood

The haem molecule with iron(II) at its centre

• Oxygen molecules are not very good ligands and bond weakly to the iron(II)
• The weak bonds allows them to break off easily and be transported into cells

Exam Hint You do not need to be familiar with the structure of the haem group

• Carbon monoxide is toxic because it is a better ligand than oxygen and binds strongly to the iron(II) preventing oxygen from being carried to the cells

  • The reason for the strength of the carbon monoxide binding more strongly to haemoglobin is that it forms stronger dative covalent bonds, although it can be displaced from heamoglobin

• If oxygen attached to the haemoglobin (oxyhaemoglobin) is replaced by carbon monoxide (carboxyhaemoglobin), a darker red colour is produced in the haem complex

  • This is a sign of carbon monoxide poisoning

• The condition anaemia occurs when a person does not have enough haemoglobin in their blood due to a loss of blood or deficiency in iron

  • Deficiency in iron can be restored by taking iron sulfate tables in the diet