Question

Why pcl5 is known and ncl5 is unknown

Answer 1

Phosphorous has d-orbitals and hence can accommodate the electrons from Cl into them, Incase of N the d-orbitals are absent and electrons can be accommodated only in p- ornitals. Hence PCl5 exists.

Answer 2

the electronic configuration of P is
${1s}^{2} {2s}^{2} {2p}^{6} {3s}^{2} {3p}^{3} {3d}^{0}$
as it has a vacant d orbital so it can extend it's covalency beyond 5
and hence can form PCl5...
(here the orbitals with n=3 has comparable energies )


but in case of NCl5...
the electronic configuration of N is
${1s}^{2} {2s}^{2} {2p}^{3}$
as N has no d orbital so it cannot extend it's covalency beyond 4....
(for d orbital to exist we need a at least n=3 but here we have upto n=2)

hence it doesn't form NCl5....


hope it helps you....
:)...