Recent structural and synthetic work on phosphorus−halo compounds over the past 3 years has provided a means of estimating associated thermochemical data. The lattice energy of Me4NPCl6 and the stability criteria for the existence of CsPCl6 as the only stable alkali metal hexachlorophosphate at ambient temperature are used to predict values for Δf H°(PCl6 -,g) while, in parallel, the rigorous calculation of the lattice potential energy of normal PCl5 (phase II, PCl5 as [PCl4 +][PCl6 -]) provides a functional thermodynamic relationship between Δf H°(PCl6 -,g) and Δf H°(PCl4 +,g) and hence enables estimation of Δf H°(PCl4 +,g). The total lattice potential energies of the [PCl4]2[PCl6]X (X = Cl (phase III, PCl5) and Br) salts are computed using a rigorous computational procedure designed to model lattices possessing complex anions and cations. These calculations lead to values of 1038 ± 5 kJ mol-1 for the lattice potential energy of [PCl4]2[PCl6]Cl and 1013 ± 5 kJ mol-1 for [PCl4]2[PCl6]Br and to estimates for the enthalpies of formation of the gaseous complex ions, Δf H°(PCl4 +,g) and Δf H°(PCl6 -,g) of 384 ± 10 kJ mol-1 and −813 ± 10 kJ mol-1, respectively. The above values differ from our previous estimates, which were based on speculative structures. Bond enthalpy estimates are made for phosphorus(V) chloro compounds. A brief consideration of the recent Glasser extension of the Kapustinskii equation is made with respect to these salts.

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