p-Block Elements (Group 15–18) · Question 9

The species formed on fluorination of phosphorus pentachloride in a polar organic solvent are

1. [PF4]+[PF6]- and [PCl4]+[PF6]-
2. [PCl4]+[PCl4F2]- and [PCl4]+[PF6]-
3. PF3 and PCl3
4. PF5 and PCl3

Answer

(2) [PCl4]+[PCl4F2]- and [PCl4]+[PF6]-

Solution

In a polar solvent, PCl5 ionises: 2PCl5 [PCl4]+[PCl6]-.

The tetrahedral cation [PCl4]+ is stable and is kept; fluorination replaces Cl by F in the octahedral anion.

Partial fluorination: [PCl6]- [PCl4F2]-, giving [PCl4]+[PCl4F2]-.

Complete fluorination of the anion: [PF6]-, giving [PCl4]+[PF6]-.

No oxidation-state change of P (+5), so PF3/PCl3 are not formed; [PF4]+ is not stable.

Answer: (B)