Ligand Substitution Reactions Overview

Introduction

• Video covers ligand substitution reactions.
• Based on AQA A-level chemistry specification.
• Important to understand definitions and concepts.

Key Concepts

• Ligands: Atoms, ions, or molecules that can donate an electron pair to form a coordinate bond.
• Coordinate Bond: Similar to a dative covalent bond.
• Complex: A central metal ion surrounded by ligands.

Types of Ligands

Monodentate Ligands

• Examples: Water (H₂O), Ammonia (NH₃), Chloride Ion (Cl⁻).
• Properties:
  • H₂O and NH₃ are similar in size and uncharged.
  • Chloride ion is larger than H₂O and NH₃.

Coordination Number

• Number of coordinate bonds in a complex.
• For H₂O and NH₃, typically 6.
• For Cl⁻, typically 4 due to size.

Ligand Substitution Reactions

Neutral Ligands (H₂O and NH₃)

• Examples:
  1. Cobalt (II) and Ammonia - Complete substitution (6 ligands).
     • Hexa Aqua Cobalt complex to Hexa Ammonia Cobalt complex.
  2. Copper (II) and Ammonia - Incomplete substitution (4 ligands).
     • Hexa Aqua Copper complex to Cu(NH₃)₄(H₂O)₂ complex.
• Color Changes:
  • Copper: Blue to Deep Blue solution.

Chloride Ligands

• Complete substitution with a decrease in coordination number (from 6 to 4).
• Equations:
  1. Copper (II) - Blue to Yellow.
  2. Cobalt (II) - Pink to Blue.
  3. Iron (III) - Yellow Brown (from deprotonation) to Yellow solution.

Biodentate Ligands

• Examples: Ethane-1,2-diamine, Ethane diorate.
• Reactions:
  • Ethane-1,2-diamine forms 3 bidentate bonds, maintaining coordination number 6.
  • Ethane diorate also forms 3 bidentate bonds, maintaining coordination number 6.

Multidentate Ligands

• Example: EDTA⁴⁻
• Forms 6 coordinate bonds per ligand.
• Example Reaction: Complete substitution of hex aqua copper complex.

Special Topics

Hemoglobin

• Key Points:
  • Heme is an Fe²⁺ complex with a multidentate ligand.
  • Oxygen forms a coordinate bond with Fe²⁺ in hemoglobin.
  • CO is toxic due to stronger bonding with hemoglobin than oxygen.

Practice Questions

1. Ligand Definition: An atom, ion, or molecule that donates an electron pair to a metal ion.
2. Incomplete Substitution: Partial replacement of ligands by another.
3. Biodentate Ligand Example: Ethane diorate, with example equations.
4. Coordination Number Change (Cl⁻): Transition from 6 to 4 due to size of chloride ions.

Conclusion

• Ensure to review and practice with the specification in mind.
• Additional topics like shapes and chelate effect to be covered separately.
• Recommended further practice and active recall for exam preparation.