ENTHALPIES OF REACTION OF ((P-CYMENE)RUCL2)(2) WITH MONODENTATE TERTIARY PHOSPHINE-LIGANDS - IMPORTANCE OF BOTH STERIC AND ELECTRONIC LIGAND FACTORS IN A RUTHENIUM(II) SYSTEM

The enthalpies of reaction of ((p-cymene)RuCl2)(2) (p-cymene = (CH3)(2)CH(C)6H(4)CH(3)) with a series of tertiary phosphine ligands, leading to the formation of (p-cymene)RuCl2(PR(3)) complexes (PR(3) = tertiary phosphine) have been measured by solution calorimetry in CH2-Cl-2 at 30 degrees C. The range of reaction enthalpies spans some 22 kcal/mol. The overall relative order of stability established is as follows (PR(3);-Delta H, kcal/mol): P(p-CF3C6H4)(3) < PCy(3) < PCy(2)Ph < P(p-ClC6H4)(3) < P(OPh)(3) < (PPr3)-Pr-i < PPh(3) < P(P-FC6H4)(3) < P(P-CH3C6H4)(3) < PCyPh(2) < P(P-CH3OC6H4)(3) < P(i)Bu(3) < PBz(3) < PPh(2)Et < PPh(2)Me < P(OMe)(3) < PEt(3) < PPhMe(2) < PMe(3). A quantitative analysis of ligand effect of the present data helps clarify the exact steric versus electronic ligand contributions to the enthalpy of reaction in this system. Both steric and electronic factors appear to play an important role in dictating the magnitude of the enthalpy of reaction.