Question

Out of $PC{l}_3,PC{l}_5,NC{l}_5$ and $NC{l}_3$, least stable is :

• A. $NC{l}_3$
• B. $PC{l}_3$
• C. $NC{l}_5$
• D. $PC{l}_5$

Answer 1

The correct option is C $NC{l}_5$

As we know, P contains vacant d-orbitals whereas N having only two energy levels doesn't have vacant d-orbitals(for principal quantum number n=2, azimuthal quantum number l = 0,1 , i.e.,only the s and p orbitals.
Under excitation the valence electrons of P can jump to the vacant d-orbitals to form the oxidation state +5 and then form $PC{l}_5$ by $s{p}^{3}d$ hybridization. However, no vacant d-orbitals in N indicates that there is no scope of expanded octet nor the formation of $NC{l}_5$.
So $NC{l}_5$ is least stable among all.