The weakest Lewis acid is

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(A) The Lewis acidity of boron trihalides $(BX_3)$ depends on the extent of $p\pi-p\pi$ back-bonding between the halogen atom and the boron atom. In $

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$BF_3$

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(A) The Lewis acidity of boron trihalides $(BX_3)$ depends on the extent of $p\pi-p\pi$ back-bonding between the halogen atom and the boron atom. In $BF_3$,the fluorine atom has a small size and its $2p$ orbitals effectively overlap with the empty $2p$ orbital of boron,leading to strong back-bonding. This reduces the electron deficiency of boron,making $BF_3$ the weakest Lewis acid. As the size of the halogen increases from $F$ to $I$,the extent of back-bonding decreases,making the boron atom more electron-deficient and thus increasing the Lewis acidity. The order of Lewis acidity is: $BF_3 < BCl_3 < BBr_3 < BI_3$.

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