Phosphorus Pentachloride - Structure

Structure

The structures for the phosphorus chlorides are invariably consistent with VSEPR theory. The structure of PCl5 depends on its environment. Gaseous and molten PCl5 is a neutral molecule with trigonal bipyramidal (D3h) symmetry. The hypervalent nature of this species (as well as for PCl−
6, see below) can be explained with three-center four-electron bonding model. This trigonal bipyramidal structure persists in non-polar solvents, such as CS2 and CCl4. In the solid state PCl5 is ionic, formulated PCl+
4PCl−
6.

In solutions of polar solvents, PCl5 undergoes "autoionization". Dilute solutions dissociate according to the following equilibrium:

PCl5 [PCl+
4]Cl−

At higher concentrations, a second equilibrium becomes more important:

2 PCl5 [PCl+
4]

The cation PCl+
4 and the anion PCl−
6 are tetrahedral and octahedral, respectively. At one time, PCl5 in solution was thought to form a dimeric structure, P2Cl10, but this suggestion is not supported by Raman spectroscopic measurements.

AsCl5 and SbCl5 also adopt trigonal bipyramidal structures. The relevant bond distances are 211 (As-Cleq), 221 (As-Cleq), 227 (Sb-Cleq), and 233.3 pm (Sb-Clax ). At low temperatures, SbCl5 converts to the dimer, bioctahedral Sb2Cl10, structurally related to niobium pentachloride.

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