Despite the much higher basicity expected for the Ru bearing the PR3 ligand in Cp(PR3)Ru(mu-Co)(2)Ru(Co)Cp, NMR studies demonstrate that protonation of this complex with CF3SO3H occurs at the Ru-Ru bond, rather than at the more basic Ru. As determined by calorimetric titration at 25.0 degrees C in 1,2-dichloroethane solvent, the enthalpy of protonation (Delta H-MHM) Of the Ru-Ru bond is higher in CP2RU2(CO)(3)(PMe3) (-30.0(4) kcal/mol) than in its carbonyl analogue Cp2Ru2(CO)(4) (-18.4(1) kcal/mol). Enthalpies (Delta H-MHM) for protonation of the Mo-Mo bond in the dinuclear Mo complexes CP2Mo2(CO)(4)(PR3)(2) show that the PMe3 complex (-27.4(2) kcal/mol) is dramatically more basic than its PMe2Ph analogue (-18.9(5) kcal/mol); Considering the Delta H-MHM values as measures of the basicities of the complexes, these results show that the basicities of metal-metal bonds are highly sensitive to the nature of their associated ligands. In addition, evidence indicates that Ru-Ru bonds are more basic than Ru in comparable mononuclear complexes. The structures, as determined by X-ray crystallographic studies, of Cp2Ru2(CO)(3)(PMe3) and its protonated derivative Cp2Ru2(CO)(3)(PMe3)(mu-H)+CF3SO3- are also discussed.