Transition metals

• Transition metal characteristics

  Arise from an incomplete d sub-level in atoms or ions
• Transition metal characteristics

  • Complex formation
  • Formation of coloured ions
  • Variable oxidation state
  • Catalytic activity
• Why Zn is not a transition metal

  Zn can only form a +2 ion, in which the Zn2+ has a complete d sublevel and so does not meet the criteria of having an incomplete d sublevel in one of its compounds
• Complex

  A central metal ion surrounded by ligands
• Ligand

  An atom, ion or molecule which can donate a lone electron pair
• Coordinate bonding

  The shared pair of electrons in the covalent bond come from only one of the bonding atoms
• Coordination number

  The number of coordinate bonds formed to a central metal ion
• Types of ligands

  • Monodentate (e.g. H2O, NH3, Cl-)
  • Bidentate (e.g. NH2CH2CH2NH2, C2O4 2-)
  • Multidentate (e.g. EDTA4-)
• Substitution reactions

  1. H2O, NH3 and Cl- can act as monodentate ligands

  2. Exchange of NH3 and H2O ligands occurs without change of coordination number

  3. Substitution may be incomplete as in the case with Cu
• Reactions with chloride ions

  1. Addition of a high concentration of chloride ions leads to a ligand substitution reaction

  2. Addition of conc HCl to aqueous ions of Cu and Co leads to a change in coordination number from 6 to 4
• Bidentate ligands

  Ligands that have two atoms with lone pairs and can form two coordinate bonds per ligand
• Bidentate ligands

  • Ethane-1,2-diamine
  • Ethanedioate ion C2O4 2-
• Formation of bidentate complexes

  [Cu(H2O)6]2+ + 3NH2CH2CH2NH2 → [Cu(NH2CH2CH2NH2)3]2+ + 6H2O

  [Cu(H2O)6]2+ + 3C2O42- → [Cu(C2O4)3]4- + 6H2O

  [Cu(H2O)6]2+ + 2C2O42- → [Cu(C2O4)2(H2O)2]2- + 4H2O
• Multidentate ligands


  Ligands that can form multiple coordinate bonds per ligand
• Multidentate ligands

  • EDTA4-
• Formation of multidentate complexes


  [Cu(H2O)6]2+ + EDTA4- → [Cu(EDTA)]2- + 6H2O
• Haem

  An iron(II) complex with a multidentate ligand that enables oxygen transport in the blood
• Chelate effect

  The substitution of monodentate ligands with a bidentate or multidentate ligand leads to a more stable complex
• The chelate effect can be explained by a positive entropy change as there are more moles of products than reactants
• EDTA titrations

  The formation of the stable EDTA complex with metal ions can be used in quantitative titrations
• Shapes of complex ions

  • Octahedral
  • Tetrahedral
  • Square planar
  • Linear
• Isomerism in complex ions

  Cis-trans isomerism and optical isomerism
• Formation of coloured ions

  Colour changes can arise from changes in oxidation state, coordination number or ligand
• How colour arises


  Colour arises from electronic transitions between different d orbitals
• Equations linking colour, wavelength, frequency and energy difference between d orbitals
• Spectrophotometry


  The amount of light absorbed is proportional to the concentration of the absorbing species
• Spectrophotometry method


  Add appropriate ligand to intensify colour

  Make up solutions of known concentration

  Measure absorption or transmission

  Plot graph of absorption vs concentration

  Measure absorption of unknown and compare
• Trends in transition metal oxidation states

  • Relative stability of +2 state increases across the period

  Compounds with high oxidation states tend to be oxidising agents

  Compounds with low oxidation states are often reducing agents
• Vanadium oxidation states


  VO2+ (+5), VO2+ (+4), V3+ (+3), V2+ (+2)
• Reduction of vanadium (V)

  Addition of zinc to vanadium (V) in acidic solution will reduce it
• Colorimetric analysis


  1. Make up solutions of known concentration

  2. Measure absorption or transmission

  3. Plot graph of absorption vs concentration

  4. Measure absorption of unknown and compare
• Transition elements


  • Show variable oxidation states
  • Lose the 4s electrons before the 3d when forming ions
• General trends in transition metal oxidation states

  • Relative stability of +2 state with respect to +3 state increases across the period
  • Compounds with high oxidation states tend to be oxidising agents e.g. MnO4
  • Compounds with low oxidation states are often reducing agents e.g. V2+ & Fe2+
• Redox potential


  Influenced by pH and by the ligand
• Vanadium oxidation states


  • VO2 + Oxidation state +5 (a yellow solution)
  • VO 2+ Oxidation state + 4 (a blue solution)
  • V 3+ Oxidation state + 3 (a green solution)
  • V 2+ Oxidation state + 2 (a violet solution)
• Reducing vanadium (V)


  Addition of zinc to the vanadium (V) in acidic solution will reduce the vanadium down through each successive oxidation state, and the colour will successively change from yellow to blue to green to violet
• VO4^3- ion


  Exists as a solid compound, usually as NH4VO4 (ammonium vanadate (V)). It is a reasonably strong oxidising agent.
• Addition of acid to the solid VO4^3-

  Turns it into the yellow solution containing the VO4^2+ ion
• Zinc metal with acid is a strong reducing agent that can reduce most transition metal ions from a higher oxidation state to the lowest state. E.g. Fe3+ to Fe2+
• Manganate redox titration


  MnO4-(aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)