Phosphorus has maximum oxidation state of +5 in PCl5. It cannot increase its oxidation state further and therefore, PCl5 cannot act as reducing agent. However, it can decrease its oxidation number from + 5 to + 3 or some lower value, therefore. PCl5 act as an oxidising agent. For example, it oxidises Ag to AgCl, Sn to SnCl4 etc.
