Solution Thermochemical Study of Tertiary Phosphine Ligand Substitution Reactions in the RhCl(CO)(PR₃)₂System

The enthalpies of reaction of [Rh(CO) 2 Cl] 2 ( 1 ) with a series of monodentate tertiary phosphine ligands, leading to the formation of RhCl(CO)(PR 3 ) 2 complexes, have been measured by anaerobic solution calorimetry in CH 2 Cl 2 at 30.0 °C. These reactions are rapid and quantitative. The measured reaction enthalpies span a range of 43 kcal/mol. The relative stability scale established is as follows: P(NC 4 H 4 ) 3 < P(NC 4 H 4 ) 2 (C 6 H 5 ) < P(OPh) 3 < P( p -CF 3 C 6 H 4 ) 3 < P(NC 4 H 4 )(C 6 H 5 ) 2 < P( p -ClC 6 H 4 ) 3 < AsEt 3 < P( p -FC 6 H 4 ) 3 < PPh 3 < P( p -CH 3 C 6 H 4 ) 3 < P( p -CH 3 OC 6 H 4 ) 3 < PPh 2 Me < P(OMe) 3 < PPhMe 2 < PEt 3 . The relative importance of phosphine electronic ligand parameters is closely examined in terms of the presented quantitative thermochemical information. Comparisons with enthalpy data in related organometallic systems are also presented.