PYTR Class

Learning Objectives

1. Outline how Lewis acids and bases are interacting in terms of electron sharing [AHL]
2. Outline the formation of complex ions [AHL]
   • What is “coordination number”?
   • What are ligands?
   • How to find the charge of the central ion in a complex ion?

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Part 1: Interactions Between Lewis Acids and Lewis Bases

The Lewis acid-base theory describes the interaction between electron pair donors (Lewis bases) and electron pair acceptors (Lewis acids). This theory is broader than the Bronsted-Lowry definition and applies to a wider range of reactions, including coordination complexes and organic reactions.

Terminologies

• Lewis acids
  • Lone pair accepters
  • They have the ability to accept electrons – which means that they are electrophiles
  • They could be:
    • Neutral – such as BF₃ where B is in the state of incomplete octet,
    • Cations – especially the transition metal ions with vacant d orbitals
• Lewis bases
  • Lone pair donors
  • They have extra at least one lone pair electrons to share and hence to form coordination bond
  • They could be:
    • Neutral – such as H₂O, NH₃
    • Anions – such as OH^–, Cl^–, CN^–

Lewis Acid-Base Interactions

Use the slide show below to see how a Lewis acid and a Lewis base interact

Part 2: Formation Complex Ions

The example above shows another interaction of lone pari sharing. In this case, it is also appropriate to refer to the species based on the terminologies:

• Lewis acid / electrophile: Al³⁺

• Lewis base / nucleophile: H₂O

This complex ion is formed when a cation accepts lone pair(s) from ligand(s).

Based on the above complex ion, 6 water molecules (the ligand) form 6 coordination bonds with a single Al³⁺ ion. This means that the coordination number is 6

What are ligands?

• Lone pair donors
• Lewis bases
• They can be:
  • Neutral: H₂O, NH₃, CO
  • Anions: F^–, Br^–, Cl^–, F^–, OH^–, SCN^–, CN^–

As you can see, a ligand is just another term used to refer a Lewis base or a nucleophile that can form a coordination bond

Part 3: Calculating Charges of Complex Ions

The overall charge of a complex ion is affected by:

• the charge on the central transition metal ion
• the charge on the ligands
• the coordination number

Using algebra, the charge of the central ion or the overall charge can be calculated.

Example: Calculate the charge of central ion in [Fe(CN)6_​]³⁻

Solution:

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Download this ppsx file to watch the tutorial

Exercises