6.2.2 Ligand Exchange

• Ligand exchange (or ligand substitution) is when one ligand in a complex is replaced by another
• Ligand exchange forms a new complex that is more stable than the original one
• The ligands in the original complex can be partially or entirely substituted by others
• The complex ion can change its charge or remain the same depending on the ligand involved
• There are no changes in coordination number, or the geometry of the complex, if the ligands are of a similar size
• But, if the ligands are of a different size, for example water ligands and chloride ligands, then a change in coordination number and the geometry of the complex will occur

Complete substitution without change in coordination number in cobalt(II) complexes

• The [Co(H₂O)₆]²⁺(aq) complex ion is pink in colour
• If ammonia solution is added to [Co(H₂O)₆]²⁺, a pale yellow / straw coloured solution will be formed
• Complete ligand substitution of the water ligands by ammonia ligands has occurred

Aqueous cobalt(II) changes to a yellow solution on addition of ammonia solution

• If excess concentrated ammonia solution is added to [Co(H₂O)₆]²⁺, a brown solution will be formed
  • The ammonia ligands make the cobalt(II) ion so unstable that it readily gets oxidised in air to cobalt(III), [Co(NH₃)₆]³⁺ (aq)
• Upon dropwise addition of sodium hydroxide (NaOH) solution to [Co(H₂O)₆]²⁺(aq), a blue precipitate is formed
• Partial ligand substitution of two water ligands by two hydroxide (OH^-) ligands has occurred

Water ligands are exchanged by hydroxide and ammonia ligands in the cobalt(II) complex

• Ligand substitution maybe incomplete if the energetics of the reaction and stability of the product are not favourable
• Copper(II)ions illustrate this behaviour with ammonia
• Different sized ligands can also lead to incomplete substitution

Incomplete substitution in copper(II) complexes

• When a transition element ion is in solution, the most common arrangement is a hexaaqua complex ion (i.e. it has six water ligands attached to it)
  • For example, Cu²⁺(aq) is [Cu(H₂O)₆]²⁺(aq)

• The [Cu(H₂O)₆]²⁺ (aq) complex ion is pale blue in colour
• Upon dropwise addition of sodium hydroxide (NaOH) solution, a light blue precipitate is formed
• Partial ligand substitution of two water ligands by two hydroxide ligands has occurred

• Upon addition of excess concentrated ammonia (NH₃) solution, the pale blue precipitate dissolves to form a deep blue solution
• Again, partial ligand substitution has occurred

• If you were to add concentrated ammonia (NH₃) solution dropwise to the [Cu(H₂O)₆]²⁺ (aq), rather than sodium hydroxide (NaOH) solution, the same light blue precipitate would form
• Again, the pale blue precipitate will dissolve to form a deep blue solution, if excess ammonia solution is then added

Addition of excess aqueous ammonia to the aqueous copper(II) ion results in a gorgeous deep blue complex

Water ligands are exchanged by hydroxide and ammonia ligands in the copper(II)  complex

Change in co-ordination number

• The water ligands in [Cu(H₂O)₆]²⁺ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid (HCl)
• The complete substitution of the water ligands causes the blue solution to turn yellow

The colour changes from light blue to a yellow-green when copper(II) is treated with concentrated hydrochloric acid. The green appearance is due to the presence of unreacted aqueous copper(II) ions

• The coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
• This is a reversible reaction, and some of the [Cu(H₂O)₆]²⁺ complex ion will still be present in the solution
  • The mixture of blue and yellow solutions in the reaction mixture will give it a green colour

• Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Cu(H₂O)₆]²⁺ (aq) ion and blue solution will return

Water ligands are exchanged by chloride ligands in the copper(II) complex

Incomplete substitution in cobalt(II) complexes

• The water ligands in [Co[H₂O)₆]²⁺ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid
• The complete substitution of the water ligands causes the pink solution to turn blue

• Like with [Cu(H₂O)₆]²⁺ above, the coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
• Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Co(H₂O)₆]²⁺ (aq) ion and pink solution will return

Water ligands are exchanged by chloride ligands in the cobalt(II) complex

• Haemoglobin is one of nature's complexes using a transition metal ion
• The haem molecule is a complex with iron(II) at its centre
• Oxygen atoms form a dative covalent bond with the Fe(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 and irreversibly to the iron(II) preventing oxygen from being carried to the cells
• If oxygen attached to the haemoglobin (oxyhaemoglobin) is replaced by carbon monoxide (carboxyhaemoglobin), a darker red colour is produced in the haem complex
  • 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

• The replacement of monodentate ligands with bidentate and multidentate ligands in complex ions is called the chelate effect
• It is an energetically favourable reaction, meaning that ΔG^ꝋ is negative
• The driving force behind the reaction is entropy
• The Gibbs equation reminds us of the link between enthalpy and entropy:

ΔG^ꝋ = ΔH_reaction^ꝋ – TΔS_system^ꝋ

• Reactions in solution between aqueous ions usually come with relatively small enthalpy changes
• However, the entropy changes are always positive in chelation because the reactions produce a net increase in the number of particles
• A small enthalpy change and relative large positive entropy change generally ensures that the overall free energy change is negative
• For example, when EDTA chelates with aqueous cobalt(II) two reactants becomes seven product species

[Co(H₂O)₆ ]²⁺ (aq) + EDTA^4- (aq) → [CoEDTA]^2- (aq) + 6H₂O (l) 

The ligand EDTA readily chelates with aqueous transition metal ions in an energetically favourable reaction