24.4 Ligand substitution and precipitation

• Ligand substitution is a reversible reaction of a complex ion in which one ligand is replaced by another ligand
• Cu2+ hexaaqua complexes react with excess aqueous ammonia by ligand substitution

  • four water molecules are replaced by four NH3 ligands
  • Colour changes from light blue to dark blue
• Cu2+ hexaaqua complexes react with an excess of chloride ions by ligand substitution

  • six water molecules are replaced by four Cl- ligands
  • Colour changes from light blue to yellow
• Cr3+ hexaaqua complexes react with excess aqueous ammonia by ligand substitution

  • six water molecules are replaced by six NH3
  • Colour changes from violet to purple
• Haemoglobin in red blood cells contain four protein chains

  • each protein chain has a planar haem molecule within its structure
  • the Fe2+ metal ion in haem bonds to a protein chain and water
  • Water can exchange readily with oxygen
• Carbon monoxide can also bond to the Fe2+ ion in haemoglobin by ligand substitution with an oxygen, the bond is so strong the process is irreversible
• A precipitation reaction is a reaction between ions in two aqueous solutions to form an insoluble ionic solid
• Aqueous transition metal ions react with NaOH and NH3 to form precipitates of the metal hydroxide -> some of these dissolve in excess to form complex ions in solution
• Cu2+ + 2OH- -> Cu(OH)2

  light blue -> blue precipitate
• Fe2+ + 2OH- -> Fe(OH)2

  pale green -> green precipitate
• The green precipitate of Fe(OH)2 turns brown at the surface because the iron(II) is oxidised to iron(III) with contact to air
• Fe3+ + 3OH- -> Fe(OH)3

  yellow -> orange brown precipitate
• Mn2+ + 2OH- -> MN(OH)2

  pale pink -> light brown precipitate
• Cr3+ + 3OH- -> Cr(OH)3

  violet -> green precipitate
• The hydroxide of Cr3+ dissolves in excess NaOH

  Cr(OH)3 -> [Cr(OH)6]3-

  green precipitate -> dark green
• The hydroxide of Cr3+ dissolves in excess NH3

  Cr(OH)3 -> [Cr(NH3)6]3+

  green precipitate -> purple
• The hydroxide of Cu2+ dissolves in excess NH3

  Cu(OH)2 -> [Cu(NH3)4(H20)2]2+

  blue precipitate -> dark blue