Question

Which one of the following statements is incorrect about the structural properties of $B{F}_3$?

• A. The unsual shortness of $B-F$ bonds is due to $p\pi -p\pi$ interaction between $B$ and $F$ atoms
• B. Double bond is delocalized giving three resonance structures.
• C. All the three $B-F$ bond lengths are equal
• D. Due to back bonding, $B{F}_3$ and $BC{l}_3$ are not isostructural molecules

Answer 1

Answer: (D)

$B{F}_3$ has a trigonal planar structure, all three $B-F$ bonds lie in the same plane. Thus, the p orbitals of Boron and Fluorine become parallel.
p-orbital of Boron is empty while fluorine has a lone pair. Fluorine donates its lone pair and this is called back bonding, or $p\pi -p\pi$ bond. Resonating structures can be obtained.
Since the $B{F}_3$ molecule is highly symmetrical, it has no dipole moment. All bonds are of equal length, thus option (B) is correct.
In Boron trihalides, the length of $B-F$ bond would be shorter than expected for a single bond, due to $p\pi -p\pi$ bonding.
$B{Cl}_3$ is also a trigonal planar molecule. Back bonding does not change the structure of the molecule. It simply decreases bond length $B-X$ and decreases Lewis acidity of $B{X}_3$.
Thus, option $\left(D\right)$ is incorrect.