Complex Ions, Ligands, and Coordination Compounds

What is a Complex Ion?

• Complex Ion: Combination of a transition metal cation with a ligand molecule or ion.
• Example: Silver metal cation (Ag+) and ammonia (NH3).
  • Silver cation + 2 ammonia molecules → Diamine silver cation.
  • Coordination Number: 2 (attached to 2 ligand molecules).
  • Geometry: Linear.

Another Example: Hexacyanoferrate Ion

• Ligand: Cyanide ion (C≡N⁻).
• Coordination Number: 6 (attached to 6 cyanide ions).
• Oxidation State of Fe:
  • Fe charge = x.
  • Reaction: Fe (x) + 6 CN⁻ → [Fe(CN)₆]³⁻.
  • Equation: x - 6 = -3 → x = +3.

Alfred Werner's Theory

• Transition metal ions have two types of valence:
  1. Primary Valence: Oxidation state (ionic bonds).
  2. Secondary Valence: Coordination number (interactions with ligands).
• Example (Fe³⁺ in [Fe(CN)₆]³⁻):
  • Primary Valence: +3
  • Secondary Valence: 6
  • Geometry: Octahedral.

Geometry Based on Coordination Numbers

• Coordination Number = 6: Octahedral geometry.
• Coordination Number = 4:
  • Tetrahedral geometry.
  • Square planar geometry.

Coordination Numbers for Transition Metal Ions

• Transition metal ion charge influences coordination number:
  • +1 charge (e.g., Cu⁺, Ag⁺): Coordination number 2 or 4 for Cu⁺, 2 for Ag⁺.
  • +2 charge (e.g., Co²⁺, Ni²⁺, Fe²⁺, Zn²⁺): Coordination number 4 or 6.
  • +3 charge (e.g., Au³⁺, Cr³⁺, Co³⁺): Coordination number 4 or 6.

Types of Ligands

• Unidentate (Monodentate) Ligands: Form one interaction per molecule with a metal ion.
  • Examples: H₂O, NH₃, Cl⁻, CN⁻, SCN⁻.
• Bidentate Ligands: Form two interactions per molecule.
  • Examples: Oxalate ion (C₂O₄²⁻), Ethylenediamine (en).
  • Bidentate ligands are chelating ligands.

Coordination Compounds

• Coordination Compound: Neutral compound consisting of a complex ion and a counter ion.
  • Example 1: [Cr(NH₃)₆]³⁺ (complex ion) and 3 CN⁻ (counter ion).
    • Charge of Cr: +3.
  • Example 2: K₃[Fe(CN)₆]
    • [Fe(CN)₆]³⁻ is the anion (complex ion).
    • K⁺ is the counter ion.

Summary

• Complex ions are formed by transition metal cations and ligands.
• Coordination number and geometry depend on the number of ligands.
• Ligands can be unidentate or bidentate.
• Coordination compounds are neutral, combining complex ions and counter ions.