Solution thermochemical study of tertiary phosphine ligand substitution reactions in the Rh(acac)(CO)(PR3) system

The enthalpies of reaction of Rh(acac)(CO)(2) (1) with a series monodentate tertiary phosphine ligands, leading to the formation of Rh(acac)(CO)(PR3) complexes, have been measured by anaerobic solution calorimetry in CH2Cl2 at 30.0 degrees C. These reactions are rapid and quantitative. The measured reaction enthalpies span a range of 12 kcal/mol. The relative stability scale established is as follows: PPh2(o-Tol) < P(p-CF3C6H4)(3) approximate to P(p-ClC6H4)(3) < P(p-FC6H4)(3) < P(NC4H4)(3) < P(NC4H4)(2)(C6H5) < P(m-CH3OC6H4)(3) < P(NC4H4)(C6H5)(2) < P(OPh)(3) approximate to PPh2(p-Tol) approximate to P(m-Tol)(3) < P(p-CH3C6H4)(3) < PPh3 < P(p-CH3OC6H4)(3) < PCy3 < PPh2Me < (PPr3)-Pr-i < PPhMe2. The relative importance of the phosphine stereoelectronic ligand parameters are examined in terms of the presented quantitative thermochemical information. Comparisons with enthalpy data in related organometallic systems are also presented.