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Among the following molecules, which one exhibits the maximum extent of p_pi - p_pi bonding?

1. NF3
2. NI3
3. BI3
4. BF3

Correct answer: BF3

Solution

In BF3, boron has an empty 2p orbital (sp2 hybrid) and each fluorine donates electron density from its filled 2p lone pairs into this orbital, creating three equivalent p_pi - p_pi back-bonds. In NF3 and NI3, nitrogen already fills its valence shell and has no vacant orbital for back-donation, while in BI3 the size mismatch between B(2p) and I(5p) makes orbital overlap negligible.

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