Ligand Substitution

• Ligand substitution- when one ligand can be swapped for another ligand.
• Substitution of similarly sized ligands:

  
• Substitution of different sized ligands:

  
• Partial substitution of ligands:

  
• Haem and haemoglobin


  • Haemoglobin is a protein found in blood that helps to transport oxygen around the body.
  • It contains Fe2+ ions, which are hexa-coordinated- six lone pairs are donated to them to form six coordinate bonds.
  • Four of the lone pairs come from nitrogen atoms, which form a circle around the Fe2+.
  • This part of the molecule is called haem.
  • The molecule that the four nitrogen atoms are part of is a multidentate ligand called a porphyrin. A protein called a globin and either an oxygen or a water molecule also bind to the Fe2+ ion to form an octahedral structure.

  
• Haem and haemoglobin (2)

  • In the body, both water and oxygen will bind to the Fe2+ ions as ligands, 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, water ligands are substituted for oxygen molecules to form oxyhemoglobin, which is carried around the body in the blood.
  • When the oxyhemoglobin gets to a place where oxygen is needed, the oxygen molecules are exchanged for water molecules and becomes deoxyhemoglobin and returns to the lungs.

  
• Carbon monoxide poisoning


  • When carbon monoxide is inhaled, the haemoglobin can substitute its water ligands for carbon monoxide ligands, forming carboxyhaemoglobin.
  • This is bad news because carbon monoxide forms a very strong bond with the Fe2+ ion and doesn't readily exchange with oxygen or water ligands, meaning the haemoglobin can't transport oxygen any more.
  • Carbon monoxide poisoning starves the organs of oxygen — it can cause headaches, dizziness, unconsciousness and even death if it's not treated.

  
• Transition metal precipitates


  When you mix an aqueous solution of transition metal ions with aqueous sodium hydroxide (NaOH) or aqueous ammonia (NH3) you get a coloured hydroxide precipitate.
• Example 1:

  
• Example 2:

  • Aqueous transition metal ions will react with ammonia to produce the same hydroxide precipitate as in their reaction with aqueous sodium hydroxide.
  • The ammonia molecules remove H+ ions from water ligands, creating OH- ligands.

  
• Example 3:

  
• Example 4: