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Substitution Reactions (Oxford AQA International A Level Chemistry)

Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Stewart Hird

Ligand Exchange

  • Ligand exchange, or substitution, is where one ligand in a complex is replaced by another

  • Ligand exchange forms a new complex that is more stable than the original one

  • During ligand exchange, ligands can undergo complete or incomplete substitution

  • This can sometimes lead to changes in:

    • The charge of the complex ion

    • The coordination number

    • The shape of the complex

Complete substitution of [Co(H2O)6]2+ (aq) by NH3

  • The [Co(H2O)6]2+(aq) complex ion is pink in colour

  • If ammonia solution is added to [Co(H2O)6]2+ (aq):

    • Incomplete substitution by hydroxide ions from the ammonia solution initially occurs forming a blue precipitate of Co(H2O)4(OH)2 (s)

    • Complete substitution by ammonia ligands forms a pale yellow / straw coloured solution of [Co(NH3)6 ]2+ (aq)

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

Diagram showing the colour change when ammonia is added to hexaaquacobalt(II)
Aqueous cobalt(II) changes to a yellow-brown solution on addition of ammonia solution
Diagram showing the ligand exchange of hexaaquacobalt(II) by ammonia
The water ligands are completely replaced by ammonia ligands

  • Complete ligand substitution of the water ligands by ammonia ligands has occurred

  • Since NH3 and H2O ligands are similar in size and uncharged, there is no change to:

    • The overall charge of the complex

    • The octahedral geometry of the complex

    • The co-ordination number of the complex

Examiner Tips and Tricks

  • 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)

Incomplete Ligand Substitution

  • There are examples where complete ligand substitution does not occur, examples include:

    • [Co(H2O)6]2+ (aq) with small amounts of dilute ammonia solution

    • [Cu(H2O)6]2+ (aq) with ammonia solution

    • [Co(H2O)6]2+ (aq), [Cu(H2O)6]2+ (aq) and [Fe(H2O)6]3+ (aq) with chloride ligands

Incomplete substitution of [Co(H2O)6]2+ (aq) by NH3

  • The [Co(H2O)6]2+(aq) complex is pink in colour

  • Initially, the dropwise addition of ammonia solution to [Co(H2O)6]2+(aq) forms a blue-green precipitate of Co(OH)2(H2O)4 (s)

    • The same reaction occurs with the addition of hydroxide ions

    • Further addition of hydroxide ions does not result in any further changes

  • Incomplete 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) 

Incomplete substitution of water ligands by hydroxide ions from ammonia solution
Incomplete substitution of water ligands by hydroxide ions from ammonia solution

  • The charge of the complex changes from +2 to 0 because two negative hydroxide ions have replaced two neutral water ligands

  • However, there is no change to:

    • The octahedral geometry of the complex

    • The co-ordination number of the complex

Incomplete substitution of [Cu(H2O)6]2+ (aq) by NH3

  • The [Cu(H2O)6]2+(aq) complex is blue in colour

  • Initially, the dropwise addition of ammonia solution to [Cu(H2O)6]2+(aq) forms a blue precipitate of Cu(OH)2(H2O)4 (s)

    • The same reaction occurs with the addition of hydroxide ions

    • Further addition of hydroxide ions does not result in any further changes

  • Incomplete ligand substitution of two water ligands by two hydroxide (OH-) ligands has occurred

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

  • The charge of the complex changes from +2 to 0 because two negative hydroxide ions have replaced two neutral water ligands

  • However, there is no change to:

    • The octahedral geometry of the complex

    • The co-ordination number of the complex

Incomplete ligand exchange with copper(II)

Diagram showing the complexes formed when hexaaquacopper(II) reacts with hydroxide ions and ammonia
Water ligands are exchanged by hydroxide and ammonia ligands in the copper(II)  complex

  • Further addition of excess concentrated ammonia (NH3) solution, causes the pale blue Cu(OH)2(H2O)4 precipitate to dissolve to form a deep blue solution

  • Again, partial ligand substitution has occurred

 [Cu(H2O)6]2+ (aq) + 4NH3 (aq) → [Cu(NH3)4(H2O)2]2+ (aq) + 2H2O (l) + 2OH– (aq)

  • There is no change to:

    • The overall charge of the complex

    • The octahedral geometry of the complex

    • The co-ordination number of the complex

Incomplete substitution by chloride ions

  • If the ligands undergoing substitution are of a different size, then:

    • The co-ordination number will change

    • The shape of the complex will change

  • Examples of different size ligands are water and the chloride ion

    • The chloride ion is larger than the water molecule, which means that less chloride ligands fit around the central metal ion

    • The chloride ion causes a change in the charge on the overall complex

  • The water ligands in [M(H2O)6]2+ (aq) and [M(H2O)6]3+ (aq) can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid (HCl)

[M(H2O)6]2+ (aq) + 4 Cl- (aq) → [MCl4]2- (aq) + 6H2O (l)

[M(H2O)6]3+ (aq) + 4 Cl- (aq) → [MCl4]- (aq) + 6H2O (l)

Chloride ions and hexaaqua cobalt(II)

  • The substitution of the water ligands causes the pink [Co(H2O)6]2+ solution to form a blue [CoCl4]2- (aq) solution

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

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

  • The following changes occur:

    • The overall charge of the complex changes from +2 to -2

    • The shape of the complex changes from octahedral to tetrahedral

    • The co-ordination number of the complex changes from 6 to 4

  • Adding water to the solution:

    • Causes the chloride ligands to be displaced by the water ligands

    • So, the pink [Co(H2O)6]2+ (aq) solution will return

Chloride ions and hexaaqua copper(II)

  • The substitution of the water ligands causes the blue [Cu(H2O)6]2+ solution to form a yellow [CuCl4]2- (aq) solution

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

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

  • The following changes occur:

    • The overall charge of the complex changes from +2 to -2

    • The shape of the complex changes from octahedral to tetrahedral

    • The co-ordination number of the complex changes from 6 to 4

  • This is a reversible reaction, and some of the [Cu(H2O)6]2+ complex 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:

    • Causes the chloride ligands to be displaced by the water ligands

    • So, the blue [Cu(H2O)6]2+ (aq) solution will return

Chloride ions and hexaaqua iron(III)

  • The substitution of the water ligands causes the yellow [Fe(H2O)6]3+ solution to form an orange [FeCl4]– (aq) solution

[Fe(H2O)6]3+ (aq) + 4 Cl- (aq) → [FeCl4]- (aq) + 6H2O (l)

Water ligands are exchanged by chloride ligands in the iron(III) complex
Water ligands are exchanged by chloride ligands in the iron(III) complex

  • The following changes occur:

    • The overall charge of the complex changes from +3 to -1

    • The shape of the complex changes from octahedral to tetrahedral

    • The co-ordination number of the complex changes from 6 to 4

  • Adding water to the solution:

    • Causes the chloride ligands to be displaced by the water ligands

    • So, the yellow [Fe(H2O)6]3+ (aq) solution will return

The Haem Complex

  • Haemoglobin is one of nature's complexes using a transition metal ion

  • The haem complex has:

    • An central metal iron(II) ion

    • A multidentate haem ligand

    • A square planar shape

  • 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 complex structure

Diagram of haemoglobin
The haem complex has 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

The effect of carbon monoxide

Diagram showing how carbon monoxide affects red blood cells
Carbon monoxide binds irreversibly to red blood cells

  • Carbon monoxide is toxic because:

    • It is a better ligand than oxygen

    • So, it binds strongly and irreversibly to the iron(II)

    • This prevents 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

    • This is a sign of carbon monoxide poisoning

The Chelate Effect

  • The chelate effect is where the monodentate ligands in a complex are replaced by bidentate and multidentate ligands

  • It is an energetically favourable reaction, i.e. ΔGꝋ is negative

  • The driving force behind the reaction is entropy

  • The Gibbs equation reminds us of the link between enthalpy and entropy:

ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ

  • Reactions in solution between aqueous ions usually come with relatively small enthalpy changes

  • However, the entropy changes are always positive in chelation

    • This is because chelation produces 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):

    • There are two reactants

    • There are seven products

    • This means that there is a large increase in entropy

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

Diagram showing the chelate effect
The ligand EDTA readily chelates with aqueous transition metal ions in an energetically favourable reaction

Examiner Tips and Tricks

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

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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.