• 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)6?]2+?(s) + 6H2O?(l)

pink solution? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?brown solution

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

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

[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

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

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

light blue solution? ? ? ? ? ? ? ? ? ? ? ? ? ? yellow solution

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

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??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)

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