Thermochemical investigation of phosphine ligand substitution reactions involving trans-(PR3)(2)Cl2Ru = CH-CH = CPh2 complexes

The enthalpies of reaction of (PPh3)(2)Cl2Ru=CH-CH=CPh2 (1) with a series o f tertiary phosphine ligands, leading to the formation of (PR3)(2)Cl2Ru=CH- CH=CPh2 complexes (PR3 = (PPr3)-Pr-i, PBz(3), PPh2Cy, PPhCy2, PCy3, and (PB u3)-Bu-i) have been measured, by solution calorimetry in CH2Cl2 at 30 degre es C. The range of reaction enthalpies spans some 6.5 kcal/mol and helps to establish a relative order of complex stability for,these six ruthenium ca rbene complexes. The enthalpies of reaction for tertiary phosphine complexe s, trans-(PR3)(2)Cl2Ru=CH-CH=CPh2, are as follows (PR3, kcal/mol): PPh3, 0; PBz(3), -1.3 (0.2); PCyPh2, -1.8 (0.2); PCy2Ph, -3.8 (0.2); (PBu3)-Bu-i, - 3.8 (0.2); (PPr3)-Pr-i, -5.2 (0.1); PCy3, -6.5 (0.3). The thermodynamics of the exchange of these sterically demanding tertiary phosphine-ligands for PPh3 in 1 provides a measurement of the relative importance of phosphine st eric and electronic properties/character to the enthalpy of reaction in thi s system. Correlations of various factors gauging the electron-donating pro perties of the phosphine ligands clearly show the electronic factor to be t he overwhelming contributor to the enthalpy of reaction in this system.