Lewis Acid–Base Reactions: Coordination Bond Formation
Let’s examine the reaction between ammonia ($ \text{NH}_3 $) and boron trifluoride ($ \text{BF}_3 $):
- The Players:
- $ \text{NH}_3 $: A Lewis base with a lone pair of electrons on nitrogen.
- $ \text{BF}_3 $: A Lewis acid because boron is electron-deficient (it has an incomplete octet).
- The Interaction:
- The lone pair on nitrogen in $ \text{NH}_3 $ is donated to the empty $ 2p_z $ orbital on boron in $ \text{BF}_3 $.
- This forms a coordination bond, resulting in $ \text{F}_3\text{B} \to \text{NH}_3 $, where the arrow indicates the direction of electron donation.
Representing the Reaction with Lewis Structures
Here’s how to represent this reaction step-by-step:
- Draw the Lewis structure for $ \text{BF}_3 $, showing boron with three single bonds to fluorine and an empty orbital.
- Draw the Lewis structure for $ \text{NH}_3 $, showing the lone pair on nitrogen.
- Use an arrow to show the lone pair from $ \text{NH}_3 $ moving toward the boron atom in $ \text{BF}_3 $.
- Draw the resulting structure, $ \text{F}_3\text{B} \to \text{NH}_3 $, with the arrow representing the coordination bond.
Example
- Consider the reaction between $ \text{AlCl}_3 $ and $ \text{Cl}^- $.
- $ \text{AlCl}_3 $ acts as a Lewis acid because the aluminum atom is electron-deficient, while $ \text{Cl}^- $ is a Lewis base with a lone pair.
- The lone pair on $ \text{Cl}^- $ is donated to $ \text{AlCl}_3 $, forming the dimer $ \text{Al}_2\text{Cl}_6 $.
Nucleophiles and Electrophiles: The Link to Lewis Acids and Bases
- In both organic and inorganic chemistry, Lewis acids and bases are often referred to as electrophiles and nucleophiles, respectively:
- Nucleophile (Lewis base): A species that donates an electron pair to form a bond. It is "nucleus-loving" and seeks positively charged or electron-deficient areas.
- Electrophile (Lewis acid): A species that accepts an electron pair to form a bond. It is "electron-loving" and seeks electron density.
- When a nucleophile reacts with an electrophile, a new bond is formed.
Example
In the reaction of $ \text{OH}^- $ with $ \text{CH}_3\text{Cl} $, the hydroxide ion donates its electron pair to the electron-deficient carbon in $ \text{CH}_3\text{Cl} $, resulting in the substitution of the chlorine atom by the hydroxide ion.
Note
While all nucleophiles are Lewis bases and all electrophiles are Lewis acids, not every Lewis acid–base pair participates in nucleophilic or electrophilic reactions.
Example
Formation of Complex Ions
- Transition metal ions, such as $ \text{Cu}^{2+} $, act as Lewis acids.
- Ligands, such as $ \text{NH}_3 $ or $ \text{H}_2\text{O} $, act as Lewis bases.
- For example, $ \text{Cu}^{2+} $ reacts with six $ \text{H}_2\text{O} $ molecules to form the complex ion $ [\text{Cu}(\text{H}_2\text{O})_6]^{2+} $, where each water molecule donates a lone pair to the $ \text{Cu}^{2+} $ ion.
Common Mistake
- Confusing Lewis acids with Brønsted–Lowry acids:
- Not all Lewis acids donate protons.
- For example, $ \text{BF}_3 $ is a Lewis acid but not a Brønsted–Lowry acid.
- Failing to show the lone pair and the arrow in Lewis structures.
- The arrow indicates the direction of electron donation, which is crucial for understanding the reaction mechanism.
Self review
- What distinguishes a Lewis acid from a Lewis base?
- How does a coordination bond differ from a traditional covalent bond?
- In the reaction between $ \text{AlCl}_3 $ and $ \text{Cl}^- $, identify the Lewis acid and Lewis base.
