Haem in Haemoglobin Contains a Multidentate Ligand

Haemoglobin is a protein found in blood that helps to transport oxygen around the body.
Haemoglobin contains Fe2+ ions, which are hexa-coordinated - six lone pairs are donated to them to form six co-ordinate bonds in an octohedral structure.
Four of the co-ordinate bonds come from a single multidentate ligand. Four nitrogen atoms from the same molecule co-ordinate around Fe2+ to form a circle. This part of the molecule is called haem.
The other two co-ordinate bonds come from a protein called globin, and either an oxygen or a water molecule - so the complex can transport oxygen to where it's needed and then swap it for a water molecule:

In the lungs, where the oxygen concentration is high, an oxygen molecule substitutes the water ligand and bonds coordinately to the Fe(II) ion to form oxyhaemoglobin, which is carried around the body in the blood.
When oxyhaemoglobin gets to a place where oxygen is needed, the oxygen molecule is exchanged for a water molecule. The haemoglobin returns to the lungs and the whole process starts again.

This process can be disrupted if carbon monoxide is inhaled. The haemoglobin swaps its water ligand for a carbon monoxide ligand, forming carboxyhaemoglobin. This is bad news because carbon monoxide is a strong ligand and doesn't readily exchange with oxygen or water ligands, meaning the haemoglobin can't transport oxygen anymore. Carbon monoxide poisoning starves the organs of oxygen - it can cause headaches, dizziness, unconsciousness and even death if it's not treated.

	Created by Lucy Nove over 9 years ago

A2 Chemistry (Transition Metals) Note on Complex Ions, created by Lucy Nove on 15/03/2017.