Examples of Ligand Exchange 

Let's take a look at some examples of ligand exchange required in A Level Chemistry syllabus.
1. Cu2+(aq) with dilute NH3
When dilute ammonia is added dropwise to Cu2+(aq), a pale blue precipitate is formed.
This is due to acid-base reaction between H2O ligands coordinated to Cu2+ and NH3.
[add nh3 dropwise]
When excess ammonia is added, the precipitate dissolves to form a deep blue complex.
This is due to ligand exchange to form complex [Cu(NH3)4(H2O)2]2+.
Take note coordination number of the copper complex remains unchanged at 6.
2. Cu2+(aq) with conc HCl
When conc HCl is added to Cu2+(aq), a yellow solution is formed.
This is due to ligand exchange to form complex [CuCl4]2-.
Take note there is a change in coordination number of the copper complex from 6 to 4.
3. O2 exchange in haemoglobin
Haemoglobin is the oxygen carrying protein in blood and the central metal is iron (II).
At the lungs where concentration of oxygen is higher, oxygen displaces water ligand to form oxyhaemoglobin and blood picks up oxygen to transport to other parts of the body.
At other parts of the body where concentration of oxygen is lower, water displaces oxygen ligand to form deoxyhaemoglobin and oxygen is released for respiration.
Therefore oyxgen and water are binded reversibly to iron (II) during normal function of haemoglobin.
In presence of carbon monoxide which is a very strong ligand, CO binds irreversibly with iron (II) which prevents haemoglobin from transporting oxygen.
Hence CO is considered poisonous.
Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore
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