• 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
• 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

Substitution in copper(II) complexes

• When a transition element ion is in solution, it can be assumed that it exists as 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?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

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

• 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?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
• The geometry of the complex has also changed from?octahedral to tetrahedral
• 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

Substitution in cobalt(II) complexes

• The [Co(H₂O)₆]²⁺(aq) complex ion is?pink?in colour
• Upon?dropwise?addition of sodium hydroxide (NaOH) solution, a?blue?precipitate is formed
• Partial?ligand substitution of two water ligands by two hydroxide (OH^-) ligands has occurred
  • If the alkali is added in?excess, the blue precipitate will turn?red?when warmed

• If?excess?concentrated?ammonia solution is added to [Co(H₂O)₆]²⁺, a?brown?solution will also be formed
  • There will be no precipitate formed in this instance, as the ammonia has been added in excess and not dropwise
• Complete?ligand substitution of the water ligands by ammonia ligands has occurred

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

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

• 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
• The geometry of the complex has also changed from?octahedral?to?tetrahedral
• 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