Microcalorimetric studies on the thermal decomposition of platinum and palladium complexes containing phosphine or diene ligands

Microcalorimetric measurements of the enthalpies of thermal decomposition of several complexes of Pt and Pd containing phosphine or diene ligands have been carried out, leading to the following values for the enthalpies of formation, ΔHf⊖(c)/kJ mol–1 of the crystalline compounds: cis-[Pt(PEt3)2Cl2]=–(655 ± 15); trans-[Pt(PEt3)2Cl2]=–(644 ± 7); trans-[Pt(PPri3)2Cl2]=–(785 ± 7); trans-[Pt{P(C6H11)3}2Cl2]=–(999 ± 8); cis-[Pd(PPh3)2Cl2]= 205 ± 10; cis-[Pt(PPh3)2Cl2]= 173 ± 6; cis-[Pt(PMe2Ph)2Cl2]=–(280 ± 5); cis[Pd(cod)Cl2]=–(234 ± 1)-(cod = cyclo-octa-1,5-diene); cis-[Pt(cod)Cl2]=–(241 ± 9); cis-[Pt(PEt3)2Me2]=–(520 ± 6); cis-[Pt(PEt3)2(C6H4Me-o)2]=–(329 ± 24); cis-[Pt(PEt3)2(CH3)Cl]=–(569 ± 5)cis-[Pt(PEt3)2(C6H4Me-o)Cl]=–(495 ± 15); cis-[Pt(PEt3)2(SEt)2]=–(651 ± 9); cis-[Pt(PPh3)2(SEt)2]= 182 ± 14; cis-[Pd-(MeCN)2Cl2]=–(149 ± 3); [Pt(PEt3)2Cl4]=–(863 ± 7). Enthalpies of sublimation of six of these compounds were measured by the vacuum-sublimation microcalorimetric technique, allowing estimates to be made for the remainder. The derived ΔHf⊖ for the gaseous complexes have served to derive bond-enthalpy contributions of L → Pt and Pt–X bonds in [PtL2X2] compounds. The ligand bonding power to PtCl2 decreases along the series PPri3 > P(C6H11)3 > PEt3 > PPh3 > olefin (in cyclo-octa-1,5-diene). Bond-enthalpy contributions of Pt–X [relative to Ē(Pt–Cl)= 290 kJ mol–1] decrease in the series Pt–C*∼ Pt–Cl > Pt–C > Pt–S, where C* and C are trigonal and tetrahedral carbon (in Ph–Pt and CH3–Pt bonds, respectively).