LIGAND AND METAL EFFECTS ON THE ENTHALPIES OF PROTONATION OF CP'M(PR3)(PR'3)X COMPLEXES (M = RU OR OS)

Titration calorimetry has been used to determine the enthalpies of protonation (DELTAH(HM)) of 22 Cp'M(PR3)(PR'3)X complexes (Cp' = eta5-C5H5(Cp) or eta5-C5Me5(Cp*); M = Ru, Os; PR3 = PPh3, PPh2Me, PPhMe2, PMe3, P(OEt)3, dppm, dppe, dppp; X = H, Cl, Br, I) with CF3SO3H in 1,2-dichloroethane solution at 25.0-degrees-C to give Cp'M(PR3)(PR'3)(X)(H)+CF3SO3-. Systematically changing the ligands and/or the metal in these complexes has yielded DELTAH(HM) values for protonation at the metal that range from -14.1 kcal/mol for CpOs(PPh3)2I to -39.2 kcal/mol for CpOs(PPh2Me)2H. Metal basicities (DELTAH(HM)) of the CpOs(PPh3)2X complexes correlate linearly with the gas-phase proton affinities of the X- ligands, both of which increase in the following order: I- < Br- < Cl- << H-. Substitution of a halide ligand with a hydride causes the metal basicity to increase by as much as 23.2 kcal/mol. The basicities of CpOs(PPh3)(PR3)Br complexes increase in the following order: P(OEt)3 < PPh3 < PMe3. There is a linear correlation between the basicities (DELTAH(HM)) of the CpOs(PR3)2Br complexes and the basicities (DELTAH(HP)) of their PR3 ligands. In a series of complexes, the Cp* ligand increases the basicity of the metal by 5.5-9.0 kcal/mol over that of the corresponding Cp derivative, and Os complexes are 6.0-8.5 kcal/mol more basic than analogous Ru complexes. Basicities of the CpOs(PR3)2(Br) and CpRu(PR3)2(H) complexes are reduced when the protonated product is contrained to have the cis, rather than trans, structure by a small-ring chelating diphosphine ligand (dppm). These studies demonstrate that the metal, ligands, and geometry of the protonated product all substantially affect the heats of protonation (DELTAH(HM)) of Cp'M(PR3)(PR'3)X complexes.