Question

Which of the following statement is/are correct regarding B-F bond in \(\mathrm{BF}_{3}\) ? (1) The unusual shortness and strength of the bonds may be explained by a p \(\pi-\mathrm{d} \pi\) interaction between boron and fluoride. (2) The unusual shortness and strength of the BF bond may be explained by \(\mathrm{p} \pi-\mathrm{p} \pi\) interaction between boron and fluorine atoms. (3) All the three B-F bond lengths are equal and each of them is shorter than the sum of the covalent radii of boron and fluoride. (4) The bond energy of the B-F bond is very high, higher than for any other single bond. (a) \(1,2,3\) (b) \(1,3,4\) (c) \(2,3,4\) (d) 1,4

Short answer

Correct statements are 2 and 3; option (c) is correct.

Step-by-step solution

Understand the Statements

First, analyze each statement to see what it implies about the B-F bond in \( \mathrm{BF}_3 \): 1. Suggests a \( p \pi-d \pi \) interaction.2. Suggests a \( p \pi-p \pi \) bond interaction.3. States that all B-F bonds are equal and shorter than the sum of the covalent radii.4. Claims the B-F bond has very high bond energy.

Analyze pπ-dπ and pπ-pπ Interactions

In \( \mathrm{BF}_3 \), boron has an empty \( p \) orbital which can overlap with \( p \) orbitals in fluorine, enabling \( p \pi-p \pi \) back bonding, making statement 2 correct. Boron does not engage in \( p \pi-d \pi \) bonding due to the absence of d orbitals, hence statement 1 is incorrect.

Compare Bond Lengths and Radii

Check if the B-F bond length is indeed shorter and equal as per statement 3. In \( \mathrm{BF}_3 \), all B-F bond lengths are equal due to resonance and are shorter than the sum of covalent radii of boron and fluorine, making statement 3 correct.

Bond Energy Consideration

Examine the bond energy: \( \mathrm{BF}_3 \) has strong B-F bonds due to the mentioned \( p \pi-p \pi \) interactions, leading to high bond energy. However, it is not higher than every other single bond, making statement 4 incorrect.

Select the Correct Statements

From Steps 2-4, the correct statements are (2) and (3). Match these with the provided options to find the correct answer choice.

Key concepts

pπ-pπ Back Bonding

In the molecule boron trifluoride, or \( \mathrm{BF}_3 \), an interesting type of bonding known as \( p\pi-p\pi \) back bonding takes place. This back bonding occurs because boron, the central atom, has an empty \( p \) orbital. Fluorine, on the other hand, possesses filled \( p \) orbitals containing lone pairs of electrons. The \( p\pi-p\pi \) back bonding can be thought of as a sort of donation of electron density from the filled \( p \) orbitals of fluorine into the empty \( p \) orbital of boron.

• This type of interaction strengthens and shortens the B-F bond by increasing the overlap between the orbitals.
• It ensures that the bond has a partial double-bond character, contributing to increased stability and strength.

Another way to view \( p\pi-p\pi \) back bonding is through the concept of resonance. It allows the electron density to be delocalized between boron and fluorine atoms, causing the B-F bond to behave differently than a typical single covalent bond.

This back bonding plays a critical role in influencing both the physical and chemical properties of \( \mathrm{BF}_3 \), explaining why statements pinpointing the \( p\pi-p\pi \) interaction, like statement 2, are deemed correct.

Covalent Bond Lengths

The covalent bond lengths in \( \mathrm{BF}_3 \) are an interesting aspect to understand. Typically, the bond length is the sum of the atomic radii of the two bonded atoms. However, in the case of \( \mathrm{BF}_3 \), each B-F bond length is shorter than the sum of boron's and fluorine's covalent radii.

• This shortening is again attributed to \( p\pi-p\pi \) back bonding, which provides a partial double-bond character to these bonds.
• Through the resonance effect, all three B-F bonds in \( \mathrm{BF}_3 \) are of equal length, even though boron could technically form one stronger bond.

This equalization of bond lengths adds to the symmetry of the molecule and contributes to its higher than expected bond energy. The reduced bond length makes \( \mathrm{BF}_3 \) more robust, confirming that statement 3 is correct about equal and shorter bond lengths relative to the sum of covalent radii.

Bond Energy Analysis

Another significant concept related to \( \mathrm{BF}_3 \) is the analysis of bond energy. Bond energy reflects the strength of the bond; higher bond energy indicates stronger bonds. In \( \mathrm{BF}_3 \), the presence of \( p\pi-p\pi \) back bonding contributes to the strength of the B-F bonds, resulting in considerable bond energy.

• While \( \mathrm{BF}_3 \)'s bond energy is substantial due to back bonding, it is not the highest among single bonds as initially suggested.
• The misconception likely arises from the enhanced stability and reduced reactivity resulting from strong partial double-bond characteristics.

Thus, it’s important to note that although the B-F bond in \( \mathrm{BF}_3 \) has high energy, claiming it surpasses every other single bond isn't accurate. The back bonding intensifies the bond, but statement 4's claim of the utmost highest bond energy isn't supported by comparative analyses of other strong bonds.