# Why is phosphorus trifluoride the strongest Lewis acid among the phosphorus trihalides?

I had read that for the trihalides of boron, boron trifluoride is the weakest Lewis acid due to backbonding. The order I had seen was -

$$\ce{BI3 > BBr3 > BCl3 > BF3}\ \ce{}$$

\$- order of acidic strength Why is the same not true for phosphorus trihalides as backbonding exists in them too?

• – Mithoron Mar 26 at 0:13