Thermochemical data for adducts of zinc, cadmium, and mercury halides with hexamethylphosphoramide

Abstract

From the standard enthalpies of dissolution of metal halides (MX₂), hexamethylphosphoramide (hmpa), and the adducts MX₂·2hmpa (M = Zn, Cd, or Hg; X = Cl, Br, or I) in an appropriate calorimetric solvent at 298.15 K, the following ΔH_R^⊖(kJ mol^–1) values for the reactions MX₂(s)+ 2hmpa(I)→ MX₂·2hmpa(s) were determined: ZnCl₂·2hmpa(s), –113.08 ± 0.79; ZnBr₂·2hmpa(s), –135.53 ± 1.18; ZnI₂2hmpa(s), –131.86 ± 0.86; CdCl₂·2hmpa(s), –55.12 ± 0.81; CdBr₂·2hmpa(s), –77.73 ± 0.67; CdI₂·2hmpa(s), –94.58 ± 0.50; HgCl₂·2hmpa(s), –63.97 ± 1.24 (ethanol), –63.01 ± 0.94 (1,2-dichloroethane); HgBr₂·2hmpa(s), –65.42 ± 1.78; and HgI₂·2hmpa, –53.41 ± 0.78. From values of ΔH_f^⊖(MX₂, s) and ΔH_f^⊖(hmpa, I) were calculated the ΔH_f^⊖(MX₂·2hmpa, s) values for the same sequence of adducts: –1 596, –1 532, –1 408, –1 514, –1 462, –1 366, –1 356, –1 304, and –1 227 kJ mol^–1. The standard enthalpies of the reactions MX₂·2hmpa(s)→ MX₂(s)+ 2hmpa(g), ΔH_D^⊖ and MX₂·2hmpa(s)→ MX₂(g)+ 2hmpa(g), ΔH_M^⊖, were also calculated and the mean dissociation enthalpy of the metal–oxygen bond, D(M–O), was estimated and compared with that for analogous adducts. The results show a slight dependence of D(M–O) on the radical attached to the phosphorus atom.