Question

Which one of the following statements is incorrect about the structural properties of BF$_3$?

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

Answer 1

Answer: (D)

(d)

Answer 2

Let's analyze each statement about the structural properties of BF$_3$.

(a) The unusual shortness of B – F bonds is due to p$\pi$ – p$\pi$ interaction between B and F atoms.
Boron in BF$_3$ is sp² hybridized and has a vacant 2p orbital. Fluorine has filled 2p orbitals. There is a possibility of back bonding (p$\pi$ – p$\pi$ interaction) between the filled 2p orbital of fluorine and the vacant 2p orbital of boron. This back donation of electron density creates a partial double bond character in the B-F bonds, which makes them shorter than a typical single bond. This statement is correct.

(b) All the three B – F bond lengths are equal.
Due to the delocalization of the double bond character through resonance involving back bonding, all three B-F bonds in BF$_3$ are equivalent and have the same bond length. This bond length is intermediate between a single bond and a double bond. This statement is correct.

(c) Double bond is delocalised giving three resonance structures.
The back bonding from fluorine to boron leads to resonance. For example, electron density from one fluorine atom's p orbital can be donated to the vacant p orbital of boron, forming a B=F bond. This results in resonance structures where the double bond character is distributed among the three B-F bonds. There are three such equivalent resonance structures, where the formal negative charge resides on boron and a formal positive charge resides on the fluorine atom involved in the double bond in each structure. This statement is correct.

(d) Due to back bonding, BF$_3$ and BCl$_3$ are not isostructural molecules.
BF$_3$ is a trigonal planar molecule with sp² hybridization on boron. BCl$_3$ is also a trigonal planar molecule with sp² hybridization on boron. Both molecules have the same molecular geometry and shape, which is trigonal planar. Therefore, BF$_3$ and BCl$_3$ are isostructural. Although back bonding occurs in both molecules (p$\pi$ - p$\pi$ between B and Cl as well), the extent of back bonding is different (more significant in BF$_3$ than in BCl$_3$). However, the presence or extent of back bonding does not change the fundamental trigonal planar structure of these molecules. The statement claims that due to back bonding, they are not isostructural, which is incorrect.

The incorrect statement is (d).