Concept: The structure of ionic compounds in the solid state often differs from their molecular form in the gaseous or liquid state. This is particularly true for compounds that can disproportionate or form complex ions to achieve greater stability in a crystal lattice.

Why (C) is correct: Phosphorus pentachloride (${\mathrm{PCl}}_5$) exists as a simple trigonal bipyramidal molecule in the gaseous and liquid phases. However, in the solid state, it undergoes autoionization (disproportionation) to form ionic species. It exists as an ionic solid composed of tetrahedral tetrachlorophosphonium cations (${\mathrm{PCl}}_4^{+}$) and octahedral hexachlorophosphate anions (${\mathrm{PCl}}_6^{-}$). This arrangement provides greater lattice energy and stability in the solid state.

Option Analysis:

• A) ${\mathrm{PCl}}_5$: This represents the molecular form of phosphorus pentachloride, which exists in the gaseous and liquid states, not the solid state.
• B) ${\mathrm{PCl}}_4^{+}{\mathrm{Cl}}^{-}$: While ${\mathrm{PCl}}_4^{+}$ is one of the ions formed, the counter-ion is not a simple chloride ion but rather the more complex ${\mathrm{PCl}}_6^{-}$ ion.
• C) ${\mathrm{PCl}}_4^{+}{\mathrm{PCl}}_6^{-}$: This correctly represents the ionic structure of solid phosphorus pentachloride, consisting of tetrachlorophosphonium cations and hexachlorophosphate anions.
• D) ${\mathrm{PCl}}_5\cdot {\mathrm{Cl}}_2$: This suggests an adduct with chlorine gas, which is not the structure of solid ${\mathrm{PCl}}_5$.

Correct Answer: (C)