This is because in PCl3 phosphorus is in the intermediate oxidising state of -3.
1. As reducing agent : The following reactions support the reducing behaviour of PCl3.
- PCl3 + SO2Cl2 → PCl5 + SO2
- PCl3 + SO3 → POCl3+ SO2
2. As an oxidising agent : It oxidises metals to their respective chlorides.
- 12Ag + 4PCl3 → 12AgCl + P4
- 6Na + PCl3 → 3NaCl + Na3P