Question

The $B—F$ bond length is the shortest in which of the following species?

• A. $B{F}_4^{-}$
• B. $B{F}_3\leftarrow N{H}_3$
• C. $B{F}_3$
• D. $B{F}_3\leftarrow N(C{H}_3{)}_3$

Answer 1

The correct option is C $B{F}_3$

Partial double bond character is developed in the $B–F$ bonds of $B{F}_3$ due to the formation of $\pi$-coordinate bond or back bond. $B{F}_3$ has a trigonal planar shape with $s{p}^2$ hybridisation.
In other cases, $B$ is $s{p}^3$ hybridised with tetrahedral shape and the possibility of $\pi$ back bonding no longer exists. Hence, the $B-F$ bond length is shortest in $B{F}_3$.