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The following was a question from the ETS Chem GRE guide and apparently this was an actual question on an exam a few decades ago. Unfortunately, no explanation for the answer was given, and I couldn't find anything more helpful on the internet.

Which of the following molecules is the strongest Lewis acid?
(a) $\ce{NF3}$ (b) $\ce{SbF5}$ (c) $\ce{NaCl}$ (d) $\ce{PCl3}$ (e) $\ce{SnCl2}$

The correct answer is supposed to be (d), which I find very surprising. I thought it should be (b), since I know that $\ce{SbF5}$ has an empty orbital that can facilitate electron donation. I also know it is a strong Lewis acid.

What orbital on $\ce{PCl3}$ is empty and why would $\ce{PCl3}$ be stronger than $\ce{SbF5}$?

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    $\begingroup$ It may be strongest base, definitely not strongest acid. $\endgroup$ – Mithoron Sep 14 '15 at 18:11
  • $\begingroup$ Related: chemistry.stackexchange.com/q/41556/44877 $\endgroup$ – Tan Yong Boon Mar 25 at 6:32
  • $\begingroup$ Such a question is really quite interesting... Typically, we would consider electron-deficient Group 3 compounds, e.g. AlCl3, BCl3 etc. as Lewis acids and would usually consider these Group 15 compounds to be Lewis bases. Considering acidity of these compounds is rather weird... $\endgroup$ – Tan Yong Boon Mar 25 at 6:37

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