17B-C. Transition metals

• Oxidation number


  The charge on a transition metal ion
• Ligand


  A molecule or ion that is attached to a transition metal ion
• Coordination number


  The number of ligands attached to a transition metal ion
• Ligand exchange


  One or more of the ligands around the transition metal ion is replaced by a different ligand
• Deprotonation


  One or more of the ligands gains or loses a hydrogen ion (proton)
• Redox


  The oxidation number of the transition metal ion changes
• Coordination number change


  The number of ligands changes
• Any one of these types of reactions can cause a change in the colour of the complex</b>
• Some reactions involve more than one of these types of reactions
• Colour changes in transition metal ions


  • Fe2+ to Fe3+
  • Cu2+ to Cu3+
  • Co2+ to Co3+
• Formation of [Cu(NH3)4(H2O)2]2+


  1. Deprotonation

  2. Ligand exchange
• Formation of [CuCl]2-


  1. Ligand exchange

  2. Coordination number change
• Amphoteric


  A substance that can act both as an acid and as a base
• Amphoteric behaviour


  The ability of a species to react with both acids and bases
• Reaction of [Co(H2O)6]2+ with aqueous NaOH


  Deprotonation
• Reaction of [Co(H2O)6]2+ with excess aqueous NH3


  1. Deprotonation

  2. Ligand exchange
• Oxidation of [Co(NH3)6]2+


  Oxidation number increases from +2 to +3, forming [Co(NH3)6]3+
• Reaction of [Co(H2O)6]2+ with concentrated HCl


  1. Ligand exchange

  2. Coordination number change
• Reaction of [Fe(H2O)6]2+ with aqueous NaOH


  Deprotonation
• No further reactions when excess aqueous NaOH or NH3 added to Fe(III) complexes
• Reaction of iron(III) complexes with alkalis


  1. Add aqueous sodium hydroxide to [Fe(H2O)6]3+ solution

  2. Yellow-brown solution forms brown precipitate

  3. [Fe(H2O)3(OH)3] + 3H2O
• Reaction of iron(III) complexes with alkalis (using ammonia)


  1. Add aqueous ammonia to [Fe(H2O)6]3+ solution

  2. [Fe(H2O)3(OH)3] + 3NH3
• The reaction [Fe(H2O)6]3+ + 30H → [Fe(H2O)3(OH)3] + 3H2O is less likely to occur than [Fe(H2O)6]3+ + 3NH3 → [Fe(H2O)3(OH)3] + 3NH4+
• Reaction of chromium(III) complexes with alkalis


  1. Add aqueous sodium hydroxide to [Cr(H2O)6]3+ solution

  2. Green solution forms green precipitate

  3. [Cr(H2O)3(OH)3] + 3H2O
• Reaction of chromium(III) complexes with alkalis (using ammonia)


  1. Add aqueous ammonia to [Cr(H2O)6]3+ solution

  2. [Cr(H2O)3(OH)3] + 3NH3 → [Cr(NH3)6]3+ + 3H2O + 30H-
• Reaction of chromium(III) complexes with excess alkali


  1. Add excess aqueous sodium hydroxide to [Cr(H2O)3(OH)3]

  2. [Cr(H2O)2(OH)4]- + H2O

  3. [Cr(OH)6]3- + 2H2O
• Chromate(VI) ions are stable in alkaline solution


  In acidic conditions, dichromate(VI) ions are more stable
• Reduction of dichromate(VI) ions with zinc in acidic conditions


  1. Cr2O7^2- + 14H+ + 3Zn → 2Cr3+ + 7H2O + 3Zn2+

  2. 2Cr3+ + Zn → 2Cr2+ + Zn2+
• Explaining oxidation of Cr3+ to Cr6+ using half-equations
• Explaining reduction of Cr6+ to Cr3+ using half-equations
• Explaining reduction of Cr3+ to Cr2+ using half-equations
• Summary of important chromium complex reactions

  • [Cr(H2O)6]3+ → [Cr(H2O)3(OH)3] → [Cr(OH)6]3-
  • [Cr(H2O)6]3+ → [Cr(NH3)6]3+
  • Cr3+ → CrO4^2- → Cr2O7^2-
  • [Cr(H2O)6]2+
• The standard electrode potential for the redox system Mn2+ + 2e = Mn is -1.18 V
• Explaining whether zinc can reduce Mn2+ ions to the element Mn
• Reaction of manganese(II) complexes with alkalis


  1. Add aqueous sodium hydroxide to [Mn(H2O)6]2+ solution

  2. Pale pink solution forms pale brown precipitate

  3. [Mn(H2O)4(OH)2] + 2H2O
• Mn2+ + 2e = Mn


  Redox system with standard electrode potential of -1.18 V
• 17B4 REACTIONS OF MANGANESE COMPLEXES
• SPECIFICATION REFERENCE 17.22 17.23
• LEARNING OBJECTIVES: Be able to record observations and write suitable equations for the reactions of Mn2+(aq) with aqueous sodium hydroxide and aqueous ammonia, including in excess. Be able to write ionic equations to show the meaning of deprotonation and ligand exchange in the reactions of manganese ions.
• Reaction of Mn2+(aq) with aqueous sodium hydroxide


  Mn(H2O)6²+ + 20OH- → Mn(H2O)4(OH)2 + 2H2O