Photolysis of RuCp(benzene)+ in CH3CN at 313 nm gives a quantitative yield of [RuCp(CH3CN)3]+ which is isolated as the PF6− salt. The quantum yield for this reaction is 0.4 ± 0.04. The thermal substitution chemistry of this complex is remarkably rich. When the reaction conditions are varied (temperature and solvent), one, two, or three of the CH3CN molecules can be selectively replaced. For example, with P(OCH3)3 as the incoming ligand, reaction of [RuCp(CH3CN)3]PF6 with an excess of P(OCH3)3 in acetonitrile at room temperature gives a quantitative yield of [RuCp(CH3CN)2(P(OCH3)3)]PF6. Further treatment of this monophosphite complex with an excess of P(OCH3)3 for several days at room temperature yields quantitatively [RuCp(CH3CN)(P(OCH3)3)2]PF6. Finally, refluxing this bisphosphite complex with an excess of P(OCH3)3 for several hours in dichloroethane yields [RuCp(P(OCH3)3)3]PF6. [CpRu(CH3CN)3]+ also reacts with a variety of unsaturated hydrocarbons to yield compounds in which all three acetonitrile molecules have been replaced. Compounds synthesized by this method include: [CpRu(η6-hexamethylbenzene)]PF6, [CpRu(η6-[2.2]-p-cyclophane)]PF6, [CpRu(η6-p-dichlorobenzene)]PF6, and [CpRu(η6-cyclooctatetraene)]PF6. Finally, photolysis of RuCp(benzene)+ in degassed CH2Cl2 in the absence of potential ligands leads to no net photoreaction. However, the addition of P(OCH3)3 or P(OEt)3 to a CH2Cl2 solution of RuCp(benzene)+ yields on photolysis RuCp(P(OR)3)3+. Photolysis of RuCp(benzene)+1 in CH2Cl2 in the presence of P(OPh)3 yields no observable products. This contrasts the behavior found for the photolysis of FeCp(p-xylene)+ which, under identical conditions, leads to the efficient formation of FeCp(P(OPh)3)3+. These results are interpreted in terms of differences in the degree of unsaturation in the reactive intermediates in the Fe and Ru systems.