The oxidation state of P in $\mathrm{PCl}_5$ is +5. As P has five electrons in itsvalence shell, it cannot increase its oxidation state beyond +5 by donatingelectrons, therefore,$\mathrm{PCl}_5$ cannot act as a reducing agent. However, it candecrease its oxidation number from +5 to +3 or some lower value, so,$\mathrm{PCl}_5$ acts as an oxidising agent. For example, it oxidises Ag to AgCl, Snto $\mathrm{SnCl}_4 .$$\begin{array}{c c c c c c c c c c c c c c c c} 0 & & +5 & & & & & & & & & & +1 & & & +3 \end{array}$ $2\mathrm{Ag} + \mathrm{PCl}_5 + \longrightarrow 2\mathrm{AgCl} + \mathrm{PCl}_3;$ $\begin{array}{cccccccccccccc} 0 & & +5 & & & & & & & +4 & & & & +3 \end{array}$$\mathrm{Sn} + 2\mathrm{PCl}_5 \longrightarrow \mathrm{SnCl}_4 + 2\mathrm{PCl}_3$