In which one of the following molecules strongest back donation of an electron pair from halide to boron is expected?
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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.
Step 1: Understanding the Question:
We need to determine in which boron trihalide (BX\(_3\)) the back donation of electrons from the halogen to the boron is the strongest. Step 2: Key Formula or Approach:
In boron trihalides, the boron atom is sp\(^2\) hybridized and has a vacant 2p orbital, making it electron-deficient. The halogen atoms have filled p orbitals containing lone pairs of electrons. Back bonding (p\(\pi\)-p\(\pi\) back donation) occurs when a lone pair from a halogen's p-orbital is donated into the empty 2p-orbital of boron.
The strength of this back bonding depends on the effectiveness of the orbital overlap. Effective overlap occurs when the interacting orbitals are of similar size and energy. Step 3: Detailed Explanation:
Let's compare the orbitals involved in back bonding for each molecule:
- BF\(_3\): Back bonding occurs between the vacant 2p orbital of Boron and a filled 2p orbital of Fluorine. Since both orbitals are in the second shell, they are of similar size and energy. This leads to very effective p\(\pi\)-p\(\pi\) overlap and strong back bonding.
- BCl\(_3\): Back bonding is between the 2p orbital of Boron and a filled 3p orbital of Chlorine. The size and energy difference between the 2p and 3p orbitals is significant, leading to less effective overlap and weaker back bonding compared to BF\(_3\).
- BBr\(_3\): Back bonding is between the 2p orbital of Boron and a filled 4p orbital of Bromine. The overlap is even less effective.
- BI\(_3\): Back bonding is between the 2p orbital of Boron and a filled 5p orbital of Iodine. The overlap is the least effective, and back bonding is the weakest.
The order of back bonding strength is: BF\(_3\)>BCl\(_3\)>BBr\(_3\)>BI\(_3\). Step 4: Final Answer:
The strongest back donation is expected in BF\(_3\) due to the most effective 2p-2p orbital overlap.
