Basicities of a series of Cp'Ru-2(2)(CO)(4) complexes were established by measuring the heats evolved (Delta(MHM)) when the complexes were protonated by CF3SO3H in 12-dichloroethane at 25.0 degrees C. Spectroscopic studies show that the protonation occurs at the metal-metal bond to form [Cp'Ru-2(2)(CO)(4)(mu-H)]+CF3SO3-, in which all of the CO ligands are terminal. The basicities (-Delta H-MHM) increase with the Cp'(2) ligands in the following order : (C-5-Me4CF3)(2) < (C9H7)(2) < C5H4C5H4 < C5H4CH2CH2C5H4 < (C5H5)(2) < (C5Me5)(2) (C5H4CH2C5H4. This trend can be understood in part by considering that more strongly donating Cp' ligands increase the basicity of the Ru-Ru bond. Another important factor is the GO-bridging or nonbridging form of each Cp'Ru-2(2)(CO)(4) complex. A dimer with bridging CO groups is significantly less basic than another dimer with only terminal CO groups although the donor abilities of their Cp' ligands are nearly equal. The Ru-Ru bond in Cp2Ru2(CO)(4) is substantially more basic than the Ru in the related mononuclear CpRu(CO)(2)H. Molecular structures of [Cp2Ru2(CO)(4)(mu-H)]+CF3SO3-, [(C5H4CH2C5H4)Ru-2(CO)(4)(mu-H)](+)-CF3SO3-, and (C5H4CH2CH2C5H4)Ru-2(CO)(4) as determined by X-ray diffraction studies are also presented.