Understanding Coordination Compounds and Their Properties

May 22, 2025

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.