JEE Advanced 2024 · Paper 2, Chemistry question 4

In a polar solvent, phosphorus pentahalides ionise into a tetrahedral $\ce{[PX4]+}$ cation and an octahedral $\ce{[PX6]-}$ anion.

Where the fluorine goes. On fluorination, F prefers the anion:

• $\ce{[PF6]-}$ is very stable;
• chlorine, the better $\pi$/lone-pair donor, stays on the cation.

Products.

• With full fluorination of the anion: $\ce{[PCl4]+[PF6]-}$.
• With partial fluorination: the mixed anion $\ce{[PCl4F2]-}$, giving $\ce{[PCl4]+[PCl4F2]-}$.

This is option (B). Options (C) and (D) are neutral molecules, not the ionic products formed in a polar solvent.

The official key accepts both (A) and (B). Option (A) pairs $\ce{[PCl4]+[PF6]-}$ with $\ce{[PF4]+[PF6]-}$.