Question

In which one of the following molecules strongest back donation of an electron pair from halide to boron is expected?

• A. BI\(_3\)
• B. BBr\(_3\)
• C. BCl\(_3\)
• D. BF\(_3\)

Hint:

The strength of back bonding in boron trihalides has a direct, but inverse, effect on their Lewis acidity. Stronger back bonding (in BF\(_3\)) makes the boron less electron-deficient, thus making BF\(_3\) the weakest Lewis acid among the trihalides. The Lewis acidity order is BI\(_3\)>BBr\(_3\)>BCl\(_3\)>BF\(_3\). This is a very common and important concept.

Answer 1

The Correct Option is D

To determine which molecule experiences the strongest back donation of an electron pair from the halide to boron, we need to understand the concept of back bonding in boron halides. Back bonding typically occurs in molecules like BF\(_3\), BCl\(_3\), BBr\(_3\), and BI\(_3\) due to the empty p-orbital of the boron atom, which can accept electron density from the filled p-orbitals of halide atoms.

The strength of back donation depends on the overlap between the orbitals and the ability of the halogen to donate electron density. Generally, the effectiveness of p\(\pi\)-p\(\pi\) back bonding is influenced by the size of the halogen and the availability of lone pair electrons:

• BF\(_3\): Fluorine is the most electronegative element and has a small size with good overlapping capacity with boron’s empty p-orbital. This makes BF\(_3\) capable of strong p\(\pi\)-p\(\pi\) back bonding.
• BCl\(_3\), BBr\(_3\), BI\(_3\): As we move from chlorine to iodine, the size of the halogen atom increases, and the ability for effective overlapping with boron’s p-orbital decreases, thereby reducing the back donation.

Among all the options, BF_3 is the molecule where the strongest back donation of an electron pair from the halide to boron is expected. This is due to the effective small size and high electronegativity of fluorine, which enhances the overlap with boron's p-orbital.

Conclusion: The correct answer is BF_3.