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

Abstract

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, ΔH_f^⊖(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 ΔH_f^⊖ 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).