(A) : B–F bond length in BF₃is less than in ${BF}_{4}^{-}$ .
(R) : Back bonding is present in BF₃ but not in ${BF}_{4}^{-}$ .

see full answer

Start JEE / NEET / Foundation preparation at rupees 99/day !!

21% of IItians & 23% of AIIMS delhi doctors are from Sri Chaitanya institute !!

An Intiative by Sri Chaitanya

(A) is true but (R) is false

Both (A) and (R) are true and (R) is the correct explanation of (A)

Both (A) and (R) are false

Both (A) and (R) are true and (R) is not the correct explanation of (A)

answer is A.

(Unlock A.I Detailed Solution for FREE)

Ready to Test Your Skills?

Check your Performance Today with our Free Mock Test used by Toppers!

Take Free Test

Detailed Solution

The B−F bond length in BF₃ is shorter than the B−F bond length in BF4-. In BF₃ bond length is 130pm, whereas in BF4- bond length is 143pm.

Explanation:

BF₃ is an electron-deficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo pπ−pπ back-bonding to remove this deficiency. Due to this a double-bond character developed to the B−F bond. This double-bond character decreases the bond length in BF₃ (130pm).

However, when BF₃ coordinates with the fluoride ion, a change in hybridization took place from sp² (in BF₃) to sp³ (in BF4-) occurs. Boron now forms 4 sigma bonds, and the double-bond character is lost. It doesn't empty 2p - orbital, so there is no back bonding. This accounts for a B−F bond length of 143pm in BF4- ion.

Watch 3-min video & get full concept clarity