Ionic Isomerism. 3. Estimation of Enthalpies of Formation of the Gaseous Tetrachlorophosphonium Ion, ΔfH°(PCl4+,g), and of the Gaseous Hexachlorophosphate Ion, ΔfH°(PCl6-,g). Lattice Enthalpy Calculations for Bis(tetrachlorophosphonium) Hexachlorophosphate Halides, [PCl4]2[PCl6]X, Where X = Cl or Br. Bond Enthalpies of Phosphorus(V) Chloro Compounds

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 Me 4 NPCl 6 and the stability criteria for the existence of CsPCl 6 as the only stable alkali metal hexachlorophosphate at ambient temperature are used to predict values for Δ f H °(PCl 6 -,g) while, in parallel, the rigorous calculation of the lattice potential energy of normal PCl 5 (phase II, PCl 5 as [PCl 4 + ][PCl 6 - ]) provides a functional thermodynamic relationship between Δ f H °(PCl 6 -,g) and Δ f H °(PCl 4 +,g) and hence enables estimation of Δ f H °(PCl 4 +,g). The total lattice potential energies of the [PCl 4 ] 2 [PCl 6 ]X (X = Cl (phase III, PCl 5 ) 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 [PCl 4 ] 2 [PCl 6 ]Cl and 1013 ± 5 kJ mol - 1 for [PCl 4 ] 2 [PCl 6 ]Br and to estimates for the enthalpies of formation of the gaseous complex ions, Δ f H °(PCl 4 +,g) and Δ f H °(PCl 6 -,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.