Question

Which of the following is the correct prediction about observed $B-F$ bond length in $B{F}_3$ molecule?

• A. $B-F$ bond length in $B{F}_3$ 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. $B{F}_3$ and ${\left[B{F}_4\right]}^{-}$ have equal $B-F$ bond length.
• C. The decrease in the $B-F$ bond length in $B{F}_3$ 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.