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 monode ntate tertiary phosphine ligands, leading to the formation of Rh(acac) (CO)(PR3) complexes, have been measured by anaerobic solution calorime try in CH2Cl2 at 30.0 degrees C. These reactions are rapid and quantit ative. The measured reaction enthalpies span a range of 12 kcal/mol. T he 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(OP h)(3) approximate to PPh2(p-Tol) approximate to P(m-Tol)(3) < P(p-CH3C 6H4)(3) < PPh3 < P(p-CH3OC6H4)(3) < PCy3 < PPh2Me < (PPr3)-Pr-i < PPhM e2. The relative importance of the phosphine stereoelectronic ligand p arameters are examined in terms of the presented quantitative thermoch emical information. Comparisons with enthalpy data in related organome tallic systems are also presented.