Transition Metals: Properties, Compounds & Applications

TAKEAWAYS:

• Transition metals are d-block elements (Groups 3–12) with partially filled d-orbitals, showing variable oxidation states and colored compounds.
• They form complexes with ligands via coordinate bonds, obeying Werner’s theory and crystal field theory.
• Their compounds act as catalysts (e.g., Fe in Haber process, Ni in hydrogenation) and pigments (e.g., Co in blue glass).
• Biological importance: Hemoglobin (Fe), chlorophyll (Mg), and vitamin B12 (Co) are essential for life.
• Alloys (e.g., steel, brass) and extraction methods (e.g., self-reduction, electrolytic refining) are key industrial applications.

1. Definition and Position in the Periodic Table

Transition metals are d-block elements (Groups 3–12) with partially filled d-orbitals in their common oxidation states. They lie between s-block (alkali/alkaline earth metals) and p-block (nonmetals).

mindmap
  root((Transition Metals))
    d-block
    Groups 3-12
    Partially filled d-orbitals
    Variable oxidation states
    Examples: Fe, Cu, Zn, Ag

Why are they called "transition metals"?

• They "transition" between highly reactive s-block metals and less reactive p-block elements.
• Their properties (color, magnetism, catalysis) are intermediate.

2. Electronic Configuration

• General formula: (n-1)d¹⁻¹⁰ ns¹⁻² (e.g., Fe: [Ar] 3d⁶ 4s²).
• Exceptions: Cr ([Ar] 3d⁵ 4s¹), Cu ([Ar] 3d¹⁰ 4s¹) due to half-filled/stable d-orbitals.
• Oxidation states: Variable (e.g., Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺) because d-electrons can be lost.

Visualize the d-orbitals filling:

3. Properties of Transition Metals

A. Physical Properties

B. Chemical Properties

1. Variable Oxidation States:

   • Due to similar energies of (n-1)d and ns orbitals.
   • Example: Mn shows +2 to +7 (MnO, MnO₂, KMnO₄).

2. Formation of Complexes:

   • Central metal ion + ligands (e.g., H₂O, NH₃, CN⁻) via coordinate bonds.
   • Example: [Cu(NH₃)₄]²⁺ (deep blue), [Fe(CN)₆]⁴⁻ (yellow).

3. Catalytic Activity:

   • Large surface area + variable oxidation states make them catalysts.
   • Examples:
     • Fe: Haber process (N₂ + H₂ → NH₃).
     • Ni: Hydrogenation of oils (C=C → C-C).
     • V₂O₅: Contact process (SO₂ → SO₃).

4. Colored Compounds:

   • d-d electron transitions absorb visible light → complementary color.
   • Example: Ti³⁺ (violet), Cr³⁺ (green), Co²⁺ (pink).

4. Complex Formation (Werner’s Theory)

Alfred Werner proposed:

• Central metal ion acts as Lewis acid (electron pair acceptor).
• Ligands act as Lewis bases (electron pair donors).
• Coordination number: Number of ligand bonds (e.g., 4 for [Ni(NH₃)₄]²⁺, 6 for [Co(NH₃)₆]³⁺).

Types of Ligands:

Example: [Co(en)₃]³⁺

• Co³⁺ (d⁶) + 3 bidentate en ligands → octahedral complex.
• More stable than monodentate ligands (chelate effect).

5. Crystal Field Theory (CFT)

Explains color and magnetism in complexes:

1. Ligands split d-orbitals into higher (eg) and lower (t₂g) energy levels in octahedral fields.
2. Δ₀ (crystal field splitting energy) determines:
   • Color: Absorption of light = Δ₀ → complementary color emitted.
   • Magnetism: Unpaired electrons → paramagnetic; paired → diamagnetic.

Visualize Δ₀ in octahedral field:

![octahedral crystal field splitting](/media/b245564d418f24857f37.png "Energy levels of d-orbitals in Ti(H₂O)₆³⁺ (purple) (Image: en:YanA, CC BY-SA 3.0, via Wikimedia Commons)")

Example: [Ti(H₂O)₆]³⁺

• Ti³⁺: d¹ configuration.
• Absorbs green light (Δ₀ ≈ 500 nm) → appears purple.

6. Important Compounds and Their Uses

7. Alloys of Transition Metals

Alloys are mixtures of metals with improved properties.

8. Extraction of Transition Metals

A. General Methods

1. Roasting: Conversion of sulfide ores to oxides (e.g., 2ZnS + 3O₂ → 2ZnO + 2SO₂).
2. Self-reduction: Impure metal reduces its oxide (e.g., Cu₂S + 2Cu₂O → 6Cu + SO₂).
3. Electrolytic refining: Impure metal as anode, pure metal deposited at cathode (e.g., Cu refining).

B. Example: Extraction of Copper

1. Ore: Chalcopyrite (CuFeS₂).
2. Steps:
   • Roasting: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂.
   • Self-reduction: Cu₂S + O₂ → 2Cu + SO₂.
   • Electrolytic refining: Impure Cu → pure Cu at cathode.

Visualize the process:

9. Biological Importance

10. Environmental and Toxic Effects

Exam Tip: How to Score Full Marks

1. Definitions:

   • Always define terms like complex, ligand, oxidation state, and alloy with examples.
   • Example: "A complex is a central metal ion bonded to ligands via coordinate bonds, e.g., [Cu(NH₃)₄]²⁺."

2. Diagrams:

   • Draw octahedral/tetrahedral complexes with ligands labeled.
   • Show crystal field splitting for d⁴–d⁷ configurations (high/low spin).

3. Applications:

   • Link properties to uses. Example: "Fe has variable oxidation states (+2/+3), making it a catalyst in the Haber process (N₂ + 3H₂ → 2NH₃)."

4. Numerical Problems:

   • NEB-style: Calculate Δ₀ from wavelength or vice versa.
     • Example: "A complex absorbs light at 500 nm. Calculate Δ₀ in kJ/mol."
     • Solution: Δ₀ = hc/λ = (6.626×10⁻³⁴ × 3×10⁸)/(500×10⁻⁹) ≈ 3.98×10⁻¹⁹ J → 240 kJ/mol.

5. Common Mistakes to Avoid:

   • Don’t confuse oxidation state (charge) with coordination number (ligand count).
   • Remember: Zn²⁺ is d¹⁰ (no unpaired electrons → diamagnetic, colorless).

NEB Board-Style Questions

Short Answer (5 marks)

1. Explain why transition metals show variable oxidation states. Give two examples with their oxidation states.
2. Draw the structure of [Co(NH₃)₄Cl₂]⁺ and name its isomers.
3. How does the Mond process refine nickel? Write the chemical equation.

Long Answer (10 marks)

1. Describe the extraction of copper from chalcopyrite. Explain the role of silica in the process.
2. Using crystal field theory, explain why [Ti(H₂O)₆]³⁺ is purple while [Ti(H₂O)₆]²⁺ is colorless.
3. Discuss the biological importance of transition metals with examples.

Solved Example: Crystal Field Stabilization Energy (CFSE)

Question: Calculate the CFSE for [CoF₆]³⁻ (Co³⁺, d⁶, weak field ligand). Solution:

1. Co³⁺: d⁶ → t₂g⁴ eg² (high spin, F⁻ is weak field).
2. CFSE = [4 × (-0.4Δ₀) + 2 × (0.6Δ₀)] = -1.6Δ₀ + 1.2Δ₀ = -0.4Δ₀.
3. Interpretation: The complex is less stable than expected due to high spin pairing energy.

Summary Table: Key Transition Metals

Final Note: Transition metals are the backbone of industrial chemistry (catalysts, alloys) and biological systems (hemoglobin, chlorophyll). Master their electronic configurations, complex formation, and applications to excel in NEB exams! Practice drawing complex structures and solving CFSE problems for full marks.