4 LIGAND SUBSTITUTION REACTIONS

• What is ligand substitution in a complex?

  It is when one ligand in a complex is replaced by another, forming a more stable complex.
• How can ligands in a complex be substituted?

  They can be partially or entirely substituted by other ligands.
• What happens to the coordination number and geometry of a complex if ligand sizes are similar?

  There is no change in the coordination number or geometry of the complex.
• What occurs if ligands of different sizes are substituted in a complex?

  The coordination number and the geometry of the complex will change.
• What is the hexaaqua complex ion?

  It is a complex ion with six water ligands attached to it.
• What is the formula for the hexaaqua complex of copper(II)?


  The formula is [Cu(H2O)6]2+(aq).
• What happens when NaOH is added dropwise to [Cu(H2O)6]2+(aq)?

  A light blue precipitate forms due to partial substitution of two H2O ligands by two OH- ligands.
• What is the ionic equation for the reaction of [Cu(H2O)6]2+(aq) with NaOH?

  [Cu(H2O)6]2+(aq) + 2OH-(aq) -> Cu(OH)2(H2O)4(s) + 2H2O(l).
• What occurs when excess concentrated ammonia (NH3) solution is added to the pale blue precipitate formed from copper(II)?


  It dissolves the precipitate to form a deep blue solution.
• What is the ionic equation for the reaction of Cu(OH)2(H2O)4(s) with NH3?

  Cu(OH)2(H2O)4(s) + 4NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH-(aq).
• What happens when concentrated ammonia (NH3) solution is added dropwise to [Cu(H2O)6]2+(aq)?

  It forms the same light blue precipitate as with NaOH.
• What occurs when excess ammonia solution is added after the light blue precipitate forms?

  The pale blue precipitate dissolves to form a deep blue solution.
• What happens to the H2O ligands in [Cu(H2O)6]2+ when concentrated HCl is added?

  They can be substituted by Cl- ligands, causing the blue solution to turn yellow.
• What is the effect of substituting H2O ligands with Cl- ligands on the coordination number and geometry of the complex?

  The coordination number changes from 6 to 4, and the geometry changes from octahedral to tetrahedral.
• What color results from mixing the blue and yellow solutions in the reaction mixture?

  The mixture gives a green color.
• What happens when H2O is added to the solution containing Cl- ligands?

  It causes the Cl- ligands to be displaced by H2O molecules, returning the [Cu(H2O)6]2+(aq) ion and blue solution.
• What color is the [Co(H2O)6]2+(aq) complex ion?

  It is pink.
• What happens when NaOH is added dropwise to [Co(H2O)6]2+(aq)?

  A blue precipitate forms due to partial ligand substitution of two H2O ligands by two OH- ligands.
• What is the ionic equation for the reaction of [Co(H2O)6]2+(aq) with NaOH?

  [Co(H2O)6]2+(aq) + 2OH-(aq) -> Co(OH)2(H2O)4(s) + 2H2O(l).
• What occurs when excess alkali is added to the blue precipitate formed from cobalt(II)?

  The blue precipitate will turn red when warmed.
• What happens when excess concentrated NH3 solution is added to [Co(H2O)6]2+?

  A brown solution is formed, with no precipitate due to complete ligand substitution of the H2O ligands by NH3 ligands.
• What is the ionic equation for the reaction of [Co(H2O)6]2+(aq) with NH3?

  [Co(H2O)6]2+(aq) + 6NH3(aq) -> [Co(NH3)6]2+(aq) + 6H2O(l).
• What happens to the cobalt(II) ion when NH3 ligands are added in excess?

  It becomes so unstable that it readily gets oxidized in air to cobalt(III), forming [Co(NH3)6]3+ and giving a brown or straw yellow solution.
• What happens to the H2O ligands in [Co(H2O)6]2+ when concentrated HCl is added?

  They can be substituted by Cl- ligands, causing the pink solution to turn blue.
• What is the ionic equation for the reaction of [Co(H2O)6]2+(aq) with Cl- ligands?

  [Co(H2O)6]2+(aq) + 4Cl-(aq) -> [CoCl4]2-(aq) + 6H2O(l).
• What is the effect of substituting H2O ligands with Cl- ligands on the coordination number and geometry of the cobalt(II) complex?

  The coordination number changes from 6 to 4, and the geometry changes from octahedral to tetrahedral.
• What happens when H2O is added to the solution containing Cl- ligands?

  It causes the Cl- ligands to be displaced by H2O molecules, returning the [Co(H2O)6]2+(aq) ion and pink solution.
• What do transition metal ions react with in aqueous solution to form precipitates?

  They react with NaOH and NH3.
• What happens to some precipitates when excess NaOH or NH3 is added?

  Some of these precipitates dissolve to form complex ions in solution.
• What is the ionic equation for the reaction of [Fe(H2O)6]2+ with NaOH?

  [Fe(H2O)6]2+(aq) + 2OH-(aq) → [Fe(H2O)4(OH)2](s) + 2H2O(l).
• What is the ionic equation for the reaction of [Cu(H2O)6]2+ with NaOH?

  [Cu(H2O)6]2+(aq) + 2OH-(aq) → [Cu(H2O)4(OH)2](s) + 2H2O(l).
• What is the ionic equation for the reaction of [Fe(H2O)6]3+ with NaOH?

  [Fe(H2O)6]3+(aq) + 3OH-(aq) → [Fe(H2O)3(OH)3](s) + 3H2O(l).
• What is the ionic equation for the reaction of [Fe(H2O)6]2+ with NH3?

  [Fe(H2O)6]2+(aq) + 2NH3(aq) → [Fe(H2O)4(OH)2](s) + 2NH4+(aq).
• What is the ionic equation for the reaction of [Cu(H2O)6]2+ with NH3?

  [Cu(H2O)6]2+(aq) + 2NH3(aq) → [Cu(H2O)4(OH)2](s) + 2NH4+(aq).
• What is the ionic equation for the reaction of [Fe(H2O)6]3+ with NH3?

  [Fe(H2O)6]3+(aq) + 3NH3(aq) → [Fe(H2O)3(OH)3](s) + 3NH4+(aq).
• How do solutions of metal aqua ions react with aqueous NH3?

  They react as acids, with some reacting further with excess NH3.