Electronic configuration of P is `1s^(2)2s^(2)2s^(6)3s^(2)3p_(x)^(1)3p_(y)^(1)3p_(z)^(1)3d^(0)`. Thus `P` has empty `3d` orbitals to which the `3s` electron can be excited to have five half-filled orbitals needed for formation of `PCI_(5)`. Thus `PCI_(5)` is known.
In contrast, electronic configuration of N is `1s^(2)2s^(2)2p_(x)^(1)2p_(y)^(1)2p_(z)^(1)`. Since the valence shell of nitrogen has `n =2` , therefore, it cannot have d-orbitals. However, if one of the `2s` electrons is excited to 3s orbital, five
half-filled orbitals needed form `NCI_(5)` can still be obtained. But such an excitation is theromodynamically not favourable since the energy needed for excitation is more than the energy expected to be gererated during the fomation to two additonal P-CI bonds. Therefore, nitrogen does not form `NCI_(5)`. In other words, `NCI_(5)` is unknown.
