In which one of the following molecules strongest back donation of an electron pair from halide to boron is expected?

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Canvas Classes · Accepted Answer

Back donation (p–p): The halide lone pair donates into the empty p-orbital of boron.

For effective back donation, the p-orbitals of B and X must have good overlap. This requires:
- Similar size of orbitals
- Good energy match

F has the smallest size among halogens and its 2p orbital has the best size match with B's empty 2p orbital → strongest p–p back donation in .

As we go down (F→Cl→Br→I), the p-orbital size increases, reducing overlap with B's 2p orbital → back donation decreases.

Order:

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