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 = η4-1,5-cyclooctadiene) with an extensive series of unidentate phosphines have been measured by solution calorimetry. The molecular structures of cis-P2PtMe2 for P = PEt3, PMe2Ph, P(pyrrolyl)3, and PCy3 have been determined by single-crystal X-ray diffraction. The relative stabilities of the resulting P2PtMe2 complexes are strongly influenced by the size (cone angle) of the incoming phosphine, with larger cone angles resulting in less thermodynamically stable complexes. Crystallographic and 31P NMR data, however, do not reflect the enthalpic stability scale and are more closely correlated to the electronic (χ) character of the phosphine ligands. The strength of the Pt−P interaction, as determined from these structural data, is greatest for phosphines with electron-withdrawing substituents, regardless of phosphine size or reaction enthalpy.