# The factor responsible for weak acidic nature of B-F bonds in BF3 is

1. A

large electronegativity of fluorine

2. B

three centred two electron bonds in BF3.

3. C

$\mathrm{p\pi }-\mathrm{d\pi }$ back bonding.

4. D

$\mathrm{p\pi }-\mathrm{p\pi }$ back bonding.

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### Solution:

BF3 is electron-deficient because B only has six valence electrons. B has one empty 2p orbital, while each F has three lone pairs of electrons. B and F have similar sizes, as a result, B and F orbitals show effective overlapping. Therefore, F can share one of its lone pairs with the unoccupied orbital of B. This results in the formation of $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding. This provides additional strength to the B-F bond and makes the B-F bond less acidic.

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