Consider the following chemical species: (I) NCl ₅ (II) [ NF ₄]⁺ (III) [ PF ₆]⁻ (IV) [ NCl ₆]⁻ Which of the given species do NOT exist specifically due to the central atom's inability to expand its valence shell?

Step-by-step solution

Nitrogen is a Period 2 element and lacks vacant d-orbitals in its valence shell. Therefore, its maximum covalency is restricted to 4 (using one 2s and three 2p orbitals). Species (II) [ NF ₄]⁺ involves nitrogen forming 4 bonds, which is within its maximum covalency limit, so it exists. Species (I) NCl ₅ and (IV) [ NCl ₆]⁻ require nitrogen to form 5 and 6 bonds respectively, which is impossible due to the absence of d-orbitals. Thus, they do not exist. Phosphorus is a Period 3 element and has vacant 3d orbitals, all