Question

Which of the following is the correct prediction about observed $B-F$ bond length in $BF_{3}$ molecule?

• A. $B-F$ bond length in $BF_{3}$ is found to be less than theoretical value because the electronegativity values of $B\left(2.04\right)$ and $F\left(4.0\right)$ suggest the bond length to be ionic and hence, the attraction between oppositely charged ions must decrease the bond length.
• B. $BF_{3}$ and $\left[BF_{4}\right]^{-}$ have equal $B-F$ bond length.
• C. The decrease in the $B-F$ bond length in $BF_{3}$ is due to delocalised $\text{p} \pi - \text{p} \pi $ bonding between vacant $2p$ orbital of $B$ and filled $2p$ orbital of $F$
• D. The correct $B-X$ bond length order is $B-F>B-Cl>B-Br>B-I$

Answer 1

Answer: (C)

It has partial double character in $B-F$ bond due to $\text{p} \pi - \text{p} \pi $ back bonding.