Question

Which of the following compounds has shortest $B-F$ bond length?

• 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 $B{F}_3$

A partial double bond character is developed in $B-F$ bonds of $B{F}_3$ due to the formation of a $p\pi$ backbonds.

In the other cases, B is $s{p}^3$ hybridized (tetrahedron molecule or ion) and the possibility for $\pi$ backbonding no longer exists.