Microcalorimetric studies of the thermal decomposition of ammonia–halide and amine–halide complexes of platinum(II), [Pt(NH2R)2X2]

Microcalorimetric measurements at elevated temperatures of the enthalpies of thermal decomposition of selected cis-[PtL2X2] complexes (L = NH3, amines, or pyridine; X = Cl, Br, or I) have been used to derive the standard enthalpies of formation, ΔHf⊖(c)/kJ mol–1, of the following crystalline compounds: [Pt(NH3)2Cl2]=–(467 ± 3); [Pt(NH3)2Br2]–(404 ± 7); [Pt(NH3)2l2]=–(295 ± 6); [Pt(NH2Me)2C12]=–(424 ± 8); [Pt(NH2Pri)2Cl2]=–(599 ± 6); [Pt(NH2Bu)2Cl2]=–(646 ± 12); [Pt(en)Cl2]=–(437 ± 7)(en = ethylenediamine); [Pt(NC5H5)2Cl2]–(21 ± 7); [Pt(NH2Pri)2Br2]=–(551 ± 13); [Pt(NH2Me)2I2]–(233 ± 8); [Pt(NH2Et)2I2]=–(334 ± 6); [Pt(NH2Pr)2I2]–(424 ± 9); [Pt(NH2Pri)2I2]=–(465 ± 7); [Pt(NH2Bui)2I2]=-(562 ± 8). Attempts to measure the enthalpies of sublimation directly were not successful. An estimated ΔHsub≈ 170 kJ mol–1 for the pyridine complex, [Pt(NC5H5)2Cl2], leads to ca. 122 kJ mol–1 for the average pyridine–platinum bond-dissociation enthalpy [=½ΔHD(g)] in the gaseous molecule. Accepting that the differences in ligand-bonding power of amines reported previously apply to amine–PtCl2 complexes, average bond-dissociation enthalpies are slightly less than with pyridine, falling to ½ΔHD(g)=ca. 97 kJ mol–1 in the case of NH3. These values imply sublimation heats of the order of 270–300 kJ mol–1 in [Pt(NH2R)2Cl2] complexes.