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

Abstract

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, ΔH_f^⊖(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 ΔH_sub≈ 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 [=½ΔH_D(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 ½ΔH_D(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.