Heterocyclic N-oxide Ligands: A Thermochemical Study Of Adducts With Zinc, Cadmium, And Mercury Chlorides

The standard enthalpy (ΔHR ⊖/kJ mol-1) of the reaction MCl2(s) + nL(s) → MCl2·nL(s) has been determined by solution calorimetry at 298.15 K, for L = pyridine N-oxide (pyo) and 2,2′-bipyridine N,N′-dioxide (bipyo) and M = Zn, Cd, and Hg: ZnCl2·2pyo, -72.1 ± 1.4; CdCl2·pyo, -24.7 ± 0.8; HgCl2·pyo, -25.7 ± 0.5; ZnCl2·1.5bipyo, -105.8 ± 1.0; CdCl2·bipyo, -24.0 ± 1.0; and HgCl2·bipyo, -4.5 ± 0.4. The standard enthalpies of formation of pyo (7.7 ± 0.8) and bipyo (16.6 ± 5.2 kJ mol-1) were obtained by solution-reaction calorimetry. The standard enthalpy of formation of the adducts decreases from zinc to mercury for both series of compounds. The enthalpy of sublimation of pyo (81.9 ± 1.5 kJ mol-1) was determined from vapour pressure measurements and that of bipyo (180 ± 10 kJ mol-1) was estimated. The mean standard enthalpies of the metal-oxygen bond were also calculated. The results show that pyo is better ligand for these metal chlorides than is bipyo.82113211