Visualizing the impact of orbital size mismatch on pπ-pπ back bonding.
1. Select Molecule
BF₃
BCl₃
BBr₃
BI₃
Lewis Acid StrengthWeak
2. Visualization Controls
3. Chemical Principles
The Anomaly: Based solely on electronegativity, one might expect BF₃ to be the strongest Lewis acid because Fluorine is the most electronegative. However, experimentally, BF₃ is the weakest, and BI₃ is the strongest.
The Explanation (Orbital Overlap):
Boron's empty orbital is a small 2p orbital. For back bonding to be effective, the halogen's filled p-orbital must be of a similar size.
• F (2p): Perfect size match. Excellent 2pπ-2pπ overlap. Boron's electron deficiency is heavily compensated.
• I (5p): Massive size mismatch. Terrible 2pπ-5pπ overlap. Back bonding is negligible, leaving Boron highly electron deficient (Strong Acid).
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