Coordination Compounds - Class 12 Chemistry

Introduction

• Coordination compounds are complex molecules consisting of a central metal atom bonded with neutral molecules or ions (ligands).
• Importance: Medicinal chemistry, industrial processes, biological systems (e.g., chlorophyll, hemoglobin, vitamin B12).

Examples

• Coordination compounds look complex but can be understood with basic concepts.
• Example:
  • Metal ion (Co) bonded with six NH3 ligands – forms complex coordination compound.
  • Coordination number: Number of coordinate bonds with central metal ion.
  • Examples: [Co(NH3)6]3+, [Fe(CN)6]4-, etc.

Werner’s Theory

• Alfred Werner (183 A): Developed theory to explain complex formation and valency concepts.
• Concepts of primary (oxidation state) and secondary (coordination number) valency.
• Primary valency: Ionizable, secondary valency: Non-ionizable, using square brackets notation.
• Experiment: Silver chloride precipitate formation to determine valency.
• Coordinating ligands, counter-ions outside square brackets: Example formulas.

Nomenclature of Coordination Compounds

1. Naming Order: Cation name first, followed by anion name.
   • E.g., K4[Fe(CN)6]: Potassium hexacyanoferrate(II).
2. Ligand First, Metal Second: Ligands named before the central metal atom.
   • E.g., [CrCl3(NH3)3]: Triamminetrichlorochromium(III).
3. No Space: In the complex name, no spaces between ligand and metal names.
4. Ligand Rules:
   • Neutral ligands named as usual (NH3 as ammine).
   • Ligands listed alphabetically.
   • Prefixes for multiple identical ligands (di-, tri-, etc.).
   • Polydentate ligands use specific prefixes (bis-, tris-).
   • Enclose polyatomic ligands in brackets.
5. Oxidation State: Indicate oxidation state of metal in Roman numerals.
   • E.g., [Cr(NH3)4Cl2]Cl: Tetraamminedichlorochromium(III) chloride.

Theories to Explain Bonding in Coordination Compounds

Valence Bond Theory (VBT)

• Hybridization: Metal ion’s valence orbitals hybridize to form coordinate bonds with ligands.
• Types of hybridizations (sp3, dsp3, d2sp3, etc.) determine geometry (tetrahedral, octahedral, etc.).
• Limitations: Doesn't explain color, magnetic properties quantitatively.

Crystal Field Theory (CFT)

• Ligands create an electrostatic field, causes splitting of metal d-orbitals into different energy levels.
• Splitting pattern depends on geometry (octahedral, tetrahedral).
• Splitting energy denoted by Δ (Δo for octahedral, Δt for tetrahedral).
• Strong field ligands cause large splitting, weak field ligands cause small splitting.
• Diagram for d-orbital splitting:
  • Octahedral: eg (higher energy), t2g (lower energy).
  • Tetrahedral splitting reversed.

Magnetic and Color Properties

• High spin complexes: Weak field ligands, small splitting, more unpaired electrons.
• Low spin complexes: Strong field ligands, large splitting, fewer unpaired electrons.
• Color: Arises due to d-d transitions, absorption of light's energy where electron jumps to a higher energy level.
• Complementary colors observed depending on the absorbed wavelength.

Applications

• Biological importance: Chlorophyll, hemoglobin, vitamin B12.
• Industrial processes: Catalysts in reactions, photographic industry.
• Medical: Cancer treatment drugs.