Ligand Substitution Reactions (OCR A Level Chemistry)

• Ligand substitution (or ligand exchange) is when one ligand in a complex is replaced by another
• Ligand substitution 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

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

• 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 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
• 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⁺ giving a brown or straw yellow solution

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

• When transition metal ions in aqueous solution react with aqueous sodium hydroxide and aqueous ammonia they form precipitates
• However, some of these precipitates will dissolve in an excess of sodium hydroxide or ammonia to form complex ions in solution

The reactions of aqueous transition metal ions with aqueous sodium hydroxide

• Examples of ionic equations for the reactions in the table above
  • [Fe(H₂O)₆]²⁺ (aq) + 2OH^- (aq) → [Fe(H₂O)₄(OH)₂] (s) +2H₂O (l)
  • [Cu(H₂O)₆]²⁺ (aq) + 2OH^- (aq) → [Cu(H₂O)₄(OH)₂] (s) +2H₂O (l)
  • [Fe(H₂O)₆]³⁺ (aq) + 3OH^- (aq) → [Fe(H₂O)₃(OH)₃] (s) + 3H₂O (l)

The reactions of aqueous transition metal ions with ammonia

• Examples of ionic equations for the reactions in the table above
  • [Fe(H₂O)₆]²⁺ (aq) + 2NH₃ (aq) → [Fe(H₂O)₄(OH)₂] (s) + 2NH₄⁺ (aq) 
  • [Cu(H₂O)₆]²⁺ (aq) + 2NH₃ (aq) → [Cu(H₂O)₄(OH)₂] (s) + 2NH₄⁺ (aq) 
  • [Fe(H₂O)₆]³⁺ (aq) + 3NH₃ (aq) → [Fe(H₂O)₃(OH)₃] (s) + 3NH₄⁺ (aq) 
  • [Cu(H₂O)₄(OH)₂] (s) + 4NH₃ (aq) → [Cu(H₂O)₂(NH₃)₄]²⁺ (aq) + 2H₂O (l) + 2OH^- (aq) 
• Solutions of metal aqua ions react as​ acids ​with aqueous ammonia, whilst some react further with excess ammonia​
• Initially, ammonia acts as a​ base​ to remove one H⁺ ion per ammonia molecule used
• With excess ammonia, some metal ions undergo ​ ligand substitution ​with NH₃