RELATIVE BINDING-ENERGIES OF STERICALLY DEMANDING TERTIARY PHOSPHINE-LIGANDS TO THE CP-ASTERISK-RUCL(CP-ASTERISK=ETA(5)-C(5)ME(5)) MOIETY -THERMOCHEMICAL INVESTIGATION OF COORDINATIVELY UNSATURATED ORGANORUTHENIUM COMPLEXES

The enthalpies of reaction of(CpRuCl)(4) (Cp* = eta(5)-C(5)Me(5)) Wit h two sterically demanding monodentate tertiary phosphine ligands, lea ding to the formation of CpRu(PR(3))Cl (PR(3) = P(C6H11)(3) and (PPr3 )-Pr-i) complexes, have been measured by anaerobic solution calorimetr y in THF at 30 degrees C. The enthalpies of reaction associated with t he rapid and quantitative reaction of the (CpRuCl)(4) complex allow f or a determination of relative ruthenium-phosphorus bond energy terms and, for the first time, enable a direct solution calorimetric measure ment of the relative donating properties of large cone-angle phosphine ligands. Reaction of CpRu-(PR(3))Cl with excess phosphine ligand, at 30 degrees C, has been shown to quantitatively yield the correspondin g CpRu(PR'(3))(2)Cl complex and allows for the design of thermochemic al cycles, assuring the internal consistency of the thermochemical dat a. The ruthenium-phosphine bond energy in CpRu(PCy(3))Cl was found to be 1.4 +/- 0.5 kcal/mol more stable than in the CpRu((PPr3)-Pr-i)Cl complex. Combining these and previously reported thermochemical data, a relative average Ru-PR(3) bond energy scale has been established: P( OMe)3 > PMe(3) > PPhMe(2) > P(OPh)3 > PPh(2)Me > PEt(3) > P(n)Bu(3) > PPh(3) > AsEt(3) > PCy(3) > (PPr3)-Pr-i. Comparisons with other organo metallic systems and insight into factors influencing the Ru-PR(3) bon d disruption enthalpies are discussed.