Synthetic, structural, and solution thermochemical studies in the dimethylbis(phosphine)platinum(II) system. Dichotomy between structural and thermodynamic trends

Reaction enthalpies of the complex (COD)PtMe2 (COD = eta(4)-1,5-cyclooctadi ene) with an extensive series of unidentate phosphines have been measured b y solution calorimetry. The molecular structures of cis-P2PtMe2 for P = PEt 3, PMe2Ph, P(pyrrolyl)(3), and PCy3 have been determined by single-crystal X-ray diffraction. The relative stabilities of the resulting P-2-PtMe2 comp lexes are strongly influenced by the size (cone angle) of the incoming phos phine, with larger cone angles resulting in less thermodynamically stable c omplexes. Crystallographic and P-31 NMR data, however, do not reflect the e nthalpic stability scale and are more closely correlated to the electronic (chi) character of the phosphine ligands. The strength of the Pt-P interact ion, as determined from these structural data, is greatest for phosphines w ith electron-withdrawing substituents, regardless of phosphine size or reac tion enthalpy.