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Problem

Compare the order of Lewis acid character of $\ce{SF6},$ $\ce{SF4}$ and $\ce{CCl4}.$

My attempt

Oxidation state of $\ce{S}$ in $\ce{SF6}$ is $+6,$ so it is very electrophilic. $\ce{S}$ has a lone pair on it in $\ce{SF4},$ but can expand its octet if attacked by a donor.

$\ce{C}$ cannot accept lone pair from donor.

So, the order should be $\ce{SF6} > \ce{SF4} > \ce{CCl4},$ but that is a wrong answer.

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    $\begingroup$ S wrapped by 6 F has not much sterical chance to reach for some external e pair donor. It is almost inert. $\endgroup$ – Poutnik Nov 27 '20 at 9:10
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S is in its highest oxidation state (+6) it has no electrophilicity hence not a Lewis acid. Also, CCl4 can't have more than 8 electrons in its outermost orbit and it can not also increase it's coordination number also so it can't behave as a Lewis acid.

SF4 can accept electrons as S isn't in its highest oxidation state.

Hence order of Lewis acid character is : SF4 > CCl4 = SF6

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