Haemoglobin & Ligand Exchange (Edexcel International A Level Chemistry)

• 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

• 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