$\mathrm{PCl}_{5} \rightarrow \mathrm{PCl}_{3}+\mathrm{Cl}_{2}$
The trigonal bipyramidal structure of phosphorus pentachloride, in which the $\mathrm{P}-\mathrm{Cl}$ (axial) bonds are somewhat larger than the $\mathrm{P}-\mathrm{Cl}$ (equatorial) bonds, renders the molecule particularly unstable. It serves as a chlorinating agent when it dissociates, releasing two chlorine atoms from the axial bond.
$\mathrm{PCl}_{5} \longrightarrow \mathrm{PCl}_{3}+\mathrm{Cl}_{2} \longrightarrow$ act as chlorinating agent