AQA A Level Chemistry

Revision Notes

6.2.2 Ligand Exchange

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 excess concentrated ammonia solution is added to [Co(H2O)6]2+, a brown solution will be formed
  • Complete ligand substitution of the water ligands by ammonia ligands has occurred

[Co(H2O)6]2+ (aq) + 6NH3 (aq) → [Co(NH3)]2+ (s) + 6H2O (l) 

pink solution                                     brown solution

Chemistry of Transition Elements - Cobalt(II) Change in Colour, downloadable AS & A Level Chemistry revision notes

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

  • The ammonia ligands make the cobalt(II) ion so unstable that it readily gets oxidised in air to cobalt(III), [Co(NH3)6]3+
  • 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

[Co(H2O)6]2+ (aq) + 2OH(aq) → Co(OH)2(H2O)4 (s) + 2H2O (l) 

pink solution                                     blue precipitate

Chemistry of Transition Elements - Ligand Exchange in Cobalt, downloadable AS & A Level Chemistry revision notes

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

Incomplete Ligand Substitution

  • 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

[Cu(H2O)6]2+ (aq) + 2OH(aq) → Cu(OH)2(H2O)4 (s) + 2H2O (l) 

blue solution                                     blue precipitate

  • 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

Cu(OH)2(H2O)4 (s) + 4NH3 (aq) → [Cu(NH3)4(H2O)2 ]2+ (aq) + 2H2O (l) + 2OH (aq) 

light blue precipitate                            deep blue solution

  • 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

Chemistry of Transition Elements - Copper(II) Change in Colour, downloadable AS & A Level Chemistry revision notes

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

Chemistry of Transition Elements - Ligand Exchange in Copper, downloadable AS & A Level Chemistry revision notes

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

[Cu(H2O)6 ]2+ (aq) + 4Cl(aq) → [CuCl4 ]2- (aq) + 6H2O (l) 

light blue solution                            yellow solution

Copper(II) chloride colour change, downloadable AS & A Level Biology revision notes

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

[Co(H2O)6 ]2+ (aq) + 4Cl(aq) → [CoCl4 ]2- (aq) + 6H2O (l) 

pink solution                            blue solution

  • 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

Chemistry of Transition Elements - Ligand Exchange in Cobalt 2_1, downloadable AS & A Level Chemistry revision notes

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

The Haem 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

Haemoglobin, downloadable AS & A Level Biology revision notes

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 cellsHydrocarbons Toxic Effect CO, downloadable AS & A Level Chemistry revision notes
  • 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 Chelate Effect

  • 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 is negative
  • The driving force behind the reaction is entropy
  • The Gibbs equation reminds us of the link between enthalpy and entropy:

ΔG = ΔHreaction – TΔSsystem

  • 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) + EDTA4- (aq) → [CoEDTA]2- (aq) + 6H2O (l) 

Chelation with EDTA, downloadable AS & A Level Biology revision notes

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

Exam Tip

Make sure you can explain the chelate effect in terms of the balance between entropy and enthalpy changes.

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