# Is phosphorus trichloride really a stronger Lewis acid than antimony pentafluoride?

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 $$\ce{SbF5}$$?

• It may be strongest base, definitely not strongest acid. – Mithoron Sep 14 '15 at 18:11
• – Tan Yong Boon Mar 25 at 6:32
• 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... – Tan Yong Boon Mar 25 at 6:37