Complexes of the type [Ru(eta(5)-C5H5)(eta(4)-C5H4O)(L)]CF3SO3 (L = CH3CN (1a), benzonitrile (1b), pyridine (2), thiourea (3)) react with tertiary phosphines to give either 1,1’- or 1,2-disubstituted ruthenocenes depending on the basicity of the entering phosphine and the nature of L. For 1a and 1b, only phosphines with a pK(a) value above 5 substitute on the C5H5 ring while others substitute on the C5H4O ring. For compounds 2 and 3, the two rings are deactivated such that only the most basic phosphines react, and they attack only the C5H4O ring. In some cases of the reactions of 2 and 3, an intermediate is observed in which the monodentate ligand has migrated to the C5H5 ring while the entering nucleophile coordinates to the metal center. The mechanism by which phosphines attack a coordinated C5H4O ring has been established, and detailed kinetic parameters have been obtained. For the reaction of 1a with PPh(3), PPh(2)Me, and P(p-PhOMe)(3) in acetone, the kinetics give a rate law indicating the reversible formation of an intermediate which goes irreversibly to the 1,2-disubstituted ruthenocene product. All three rate constants and their thermal activation parameters have been obtained for each of these reactions. For the P(p-PhOMe)(3) reaction, the volume of activation for each step has also been determined. The reaction of 2 and 3 with PBu(3)(n), PCy(3), PPhMe(2), and PMe(3) in CD3CN give a long-lived intermediate which also goes to the 1,2-disubstituted ruthenocene product. For the intermediates formed from 3, the kinetics of this last step have been studied to determine the rate constants and their thermal activation parameters. In the case of PBu(3)(n), the intermediate formed from 3 has been isolated and an X-ray structure determined, establishing that phosphine attack has occurred at the C5H4O ring.