1. NCl5 is unstable
2. larger size of nitrogen
3. inertness of nitrogen
4. non-availability of vacant d- atomic orbitals
Solution: non-availability of vacant d- atomic orbitals
By employing the d-orbitals to “expand the octet” and having additional places to insert bonding pairs of electrons, PCl5 makes five bonds. Because there are no d-orbitals at the second energy level, NCl5 does not exist. As a result, arranging five pairs of bonding electrons around a nitrogen atom is impossible.