Heterocyclic nitrogen-containing electron-pair donor ligands: a thermochemical study of adducts with zinc, cadmium, and mercury chlorides

Abstract

The standard enthalpies (ΔH_R^⊖/kJ mol^–1) of the reactions MCl₂(s)+nL(s,l)→ MCl₂·nL(s) for ligands pyridine (py), 2,2′-bipyridine (bipy), and 1,10-phenanthroline (phen) were measured calorimetrically at 298.15 K, giving: ZnCl₂·2py, –116.1 ± 1.0; CdCl₂·py, –46.8 ± 0.5; HgCl₂·py, –43.4 ± 0.4; ZnCl₂·bipy, –82.9 ± 0.5; CdCl₂·bipy, –53.9 ± 1.6; HgCl₂·bipy, –37.7 ± 0.9; ZnCl₂·phen, –83.5 ± 1.5; CdCl₂·phen, –61.7 ± 1.7; and HgCl₂·phen, –60.0 ± 1.6. The standard enthalpy of formation of bipy (216.4 ± 7.4 kJ mol^–1) was derived by combustion calorimetry, while that for phen (354 kJ mol^–1) was obtained by an estimative calculation method. The standard enthalpies of formation of the adducts were calculated, showing a decrease from zinc to mercury, and for each metal chloride the decrease py > bipy > phen was observed. The standard enthalpies of sublimation of the ligands bipy and phen were calculated, giving 67 and 65 kJ mol^–1, respectively. The standard enthalpy of the metal–nitrogen bond, D(M–N), was calculated by assuming the equivalence of each bond in a bidentate ligand. Values of D(M–N) for adducts of bidentate ligands were lower than the corresponding ones of pyridine.