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  4. The strongest Lewis acid among boron halides is

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Q. The strongest Lewis acid among boron halides is

AMUAMU 2010The p-Block Elements

Solution:

Generally, as the electronegativity increases, acidic character increases but in case of boron halides, $BI _{3}$ is most acidic, ie, strongest Lewis acid. This is because there is back donation of electrons from $F$ atom to boron in case of $BF _{3}$. This makes the $BF _{3}$ less electron deficient. The tendency of back donation decreases as the size of halogen atom increases (due to large energy difference). Hence, $BI _{3}$ is most electron deficient and thus, the strongest Lewis acid.