AQA Chemistry A-Level: Transition Metals

3.2.5.1 Transition Metal Properties

• Transition Metals: Elements in the d-block with a partially filled d-orbital.
  • Lose electrons; s-orbital electrons removed first.
  • Similar physical properties: atomic radius, high densities, melting and boiling points.
  • Special chemical properties:
    • Form complexes
    • Form coloured ions
    • Variable oxidation states
    • Good catalysts

Complexes

• Formed when transition metals are surrounded by ligands.
• Represented in formulas with square brackets.

Ligands

• Molecules/ions with lone electron pairs; form coordinate bonds with metal ions.
• Common ligands: Cl-, H2O, NH3
• Coordination Number: Number of coordinate bonds around a metal ion (e.g., Silver - 2, Platinum - 4).

3.2.5.2 Ligand Substitution Reactions

• NH3 and H2O: Same size, uncharged; can exchange without changing coordination number.
  • E.g., Copper complexes with NH3 form deep blue solutions.
• Cl-: Larger, results in a change in coordination number; complexes with Cl- always form tetrahedral shapes.

Bidentate Ligands

• Form two coordinate bonds (e.g., Ethanedioate ions, Ethane-1,2-diamine).
• Replace two unidentate ligands, maintaining coordination number.

Multidentate Ligands

• Form up to six bonds (e.g., EDTA, Haem).
• Chelate Effect: Favourable positive entropy change from substituting unidentate ligands with bidentate/multidentate ones.
• Enthalpy change for ligand substitutions is small.

3.2.5.3 Shapes of Complex Ions

• Octahedral Complexes: Common with H2O and NH3 ligands; 90° bond angle.
  • Cis-trans Isomerism: Shown by octahedral complexes.
  • Optical Isomerism: Shown by octahedral complexes with bidentate ligands.
• Tetrahedral Complexes: Formed with larger ligands like Cl-; 109.5° bond angle.
• Square Planar Complexes: Formed by Platinum and Nickel; 90° bond angle.
• Linear Complexes: Silver complexes; 180° bond angle (e.g., Tollens reagent).

Cisplatin

• Cis isomer of a square planar platinum complex used in cancer therapy.

3.2.5.4 Coloured Ions

• Colour depends on coordination number, ligand type, oxidation state.
• Colour results from absorption and reflection of light wavelengths.
• Colorimetry: Technique to determine concentration of coloured ions by measuring absorbance.

3.2.5.5 Variable Oxidation States

• Transition metals have variable oxidation states (e.g., Vanadium states from +5 to +2).
• Colour variations with oxidation state changes.
• pH affects oxidation/reduction; acidic conditions for reduction, alkaline for oxidation.

Tollens Reagent

• Silver complex [Ag(NH3)2]+ used to test for aldehydes.

3.2.5.6 Catalysts

• Catalysts speed up reactions without being consumed.
• Heterogeneous Catalysts: Different phase from reactants; e.g., Iron in Haber Process.
  • Variable oxidation states aid catalysis (e.g., Vanadium in the Contact Process).
  • Adsorption: Molecules adsorbed on catalyst surface, increasing reaction rate.
  • Catalyst poisoning by impurities can increase production costs.

Homogeneous Catalysts

• Same phase as reactants; e.g., Fe2+ in reaction with S2O8 2- and I- ions.

Autocatalysis

• Reaction product acts as a catalyst; e.g., MnO4- and C2O42- reactions where Mn2+ catalyzes further reaction.