In case of nitrogen, NCI₃ is possible but not NCl₅ while in case of phosphorous, PCl₃ as well as PCl₅ are possible. It is due to

Both N and P are members of group-15 of the periodic table. N is the first member and has an electronic configuration $1{\mathrm{s}}^2 2{\mathrm{s}}^2 2{\mathrm{p}}^3.$ It forms a maximum of three bonds because its valency is 3. It has no vacant d orbitals, so it cannot extend its valency.

P is the second member of group 15 and its valance shell electronic configuration is $3{\mathrm{s}}^2 3{\mathrm{p}}^3.$ Its valency is 3, hence forms three bonds with neighboring atoms. But as P has vacant 3d orbitals, it can extend its valency. For instance, in PCl₃ (showing valency 3 of phosphorus) and PCl₅ (showing valency 5 of phosphorus).

Hence phosphorous can form pentahalides but nitrogen can not form pentahalides