Q.$\text{PCl}_5$ is trigonal bipyramidal in the gas phase but behaves as an ionic solid in the crystalline state. Describe both structures and explain the difference.

[!TLDR] Gas-phase PCl5 is trigonal bipyramidal (sp3d P, 3 equatorial + 2 longer axial Cl); the solid is ionic, tetrahedral [PCl4]+ paired with octahedral [PCl6]-. [!ANSWER] PCl5 vapour is trigonal bipy …

In the gas/vapour phase, $\text{PCl}_5$ is a discrete, covalent molecule: phosphorus is $sp^3d$ hybridised, with three chlorine atoms in equatorial positions at $120^\circ$ to each other and two chlorine atoms in axial positions, perpendicular to the equatorial plane, giving a trigonal bipyramidal shape. The two axial P-Cl bonds are experimentally longer and weaker than the three equatorial bonds, because each axial bond is repelled by three neighbouring equatorial bonding pairs at $90^\circ$, while each equatorial bond is repelled by only two axial pairs at $90^\circ$ (plus two equatorial pairs at the wider $120^\circ$). In the solid state, $\text{PCl}_5$ instead adopts an ionic structure: one $\text{PCl}_5$ unit effectively donates a chloride ion to a second $\text{PCl}_5$ unit, giving a lattice of tetrahedral $[\text{PCl}_4]^+$ cations ($sp^3$ phosphorus, four equivalent Cl at $109.5^\circ$) paired with octahedral $[\text{PCl}_6]^-$ anions ($sp^3d^2$ phosphorus, six equivalent Cl at $90^\circ$). This gas-versus-solid structural change happens because the ionic lattice, with its strong ele …

Describe the covalent gas-phase geometry first (trigonal bipyramidal, sp3d, note the axial/equatorial bond-length difference), then describe the separate ionic solid-state structu …

Describing PCl5 as having only one structure; assuming the solid-state ions have the same shape as the gas-phase neutr …