6.3.1 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
• 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