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)PtMe<sub>2</sub> (COD = η<sup>4</sup>-1,5-cyclooctadiene) with an extensive series of unidentate phosphines have been measured by solution calorimetry. The molecular structures of <i>cis</i>-P<sub>2</sub>PtMe<sub>2</sub> for P = PEt<sub>3</sub>, PMe<sub>2</sub>Ph, P(pyrrolyl)<sub>3</sub>, and PCy<sub>3</sub> have been determined by single-crystal X-ray diffraction. The relative stabilities of the resulting P<sub>2</sub>PtMe<sub>2</sub> 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 <sup>31</sup>P 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.