Coordination Compounds Lecture Notes

Introduction to Coordination Compounds

• D Block Elements: Examples include iron, copper, zinc.
  • These metals typically lose electrons, but in coordination compounds, they accept electron pairs.
• Reasons Metals Accept Electron Pairs:
  1. Metals have empty shells.
  2. Metals achieve stability by completing the octet rule.

Formation of Coordination Compounds

• Example: Copper and NH₃.
  • Copper (metal atom) accepts a pair of electrons from NH₃ (neutral molecule with a lone pair).
  • Coordinate Bond: Formed between NH₃ and copper.

Definitions

• Coordination Compounds: Complex compounds where a metal atom is bonded to ions or molecules.
• Ligand: An ion or molecule that donates a pair of electrons to a central metal atom.
• Composition: Metal atom + ligand(s).

Basic Terms

• Central Atom: The metal atom in a coordination compound (e.g., chromium in [Cr(NH₃)₆]Cl₃).
• Coordination Number: Number of electron pairs donated (e.g., 6 for [Cr(NH₃)₆]Cl₃).
• Coordination Sphere: The central atom and ligands enclosed in brackets.
• Counter Ion: Ion outside the coordination sphere (e.g., Cl in [Cr(NH₃)₆]Cl₃).

Werner’s Theory

• Werner: First to study coordination compounds.
  • Developed [Co(NH₃)₆]Cl₃, showing different types of bonding with ammonia and chloride.
• Types of Valencies:
  1. Primary Valency: Satisfied by negative ions; represents oxidation state; ionizable.
  2. Secondary Valency: Satisfied by neutral molecules or positive ions; represents coordination number; non-ionizable.

Calculating Valencies

• Primary Valency: Oxidation number of central atom.
• Secondary Valency: Coordination number.

Structure and Examples

• Werner’s Structures: Showcases positioning of primary and secondary valencies.

Types of Ligands

• Denticity: Number of lone pairs donated by a ligand.
• Types of Ligands:
  1. Unidentate/Monodentate: Donate one pair (e.g., Cl⁻).
  2. Bidentate: Donate two pairs (e.g., ethylenediamine - en).
  3. Ambidentate: Can donate via different atoms (e.g., SCN⁻).
  4. Polydentate: Donate multiple pairs (e.g., EDTA).

Complexes

• Homoleptic Complex: Single type of ligand.
• Heteroleptic Complex: Multiple types of ligands.

Nomenclature of Coordination Compounds

• Ligand Names:
  • Neutral: e.g., NH₃ (ammine), H₂O (aqua).
  • Anionic: Add "-o" (e.g., Cl⁻ = chloro).
  • Cationic: Add "-ium" (e.g., NO⁺ = nitrosonium).
• Naming Rules:
  1. Left to right.
  2. Alphabetical order for ligands.
  3. Add -ate for negative coordination spheres.

Crystal Field Theory (CFT)

• Concept: Explains splitting of d-orbitals when ligands approach a metal.
• Orbital Splitting:
  • Octahedral: Ligands attack along axes; splits into e_g and t_2g.
  • Tetrahedral: Ligands attack between axes.
• Energy Levels: Different for e_g and t_2g in octahedral; reversed in tetrahedral.

Valence Bond Theory (VBT) and Hybridization

• Hybridization: Process of orbitals combining to form new hybrid orbitals.
• Examples:
  • Inner Orbital Complex: e.g., d²sp³ (octahedral).
  • Magnetic Properties: Depend on pairing of electrons (low-spin vs. high-spin).

Summary

• Coordination compounds involve metal atoms accepting electron pairs from ligands.
• Werner's work laid the foundation for understanding structure and valency.
• CFT and VBT provide insights into bonding and properties of coordination compounds.