The synthesis and characterization of new [(η5-C5H5)Ru(η6-arene)]PF6 and [(η5-C5(CH3)5)Ru(η6-arene)]PF6 complexes are reported (arene = naphthalene, anthracene, pyrene, chrysene, azulene). Kinetic studies of arene displacement by acetonitrile for five of these complexes are reported. The values obtained for &obsd range from 1.3 X 10-2 s-1 to 4.6 X 10-6s-1 in 2.73 M CH3CN. The differences between the rates observed for the C5H5 - complexes and the C5(CH3)5 - compound are explained in terms of an associative mechanism. Rate constants for the ruthenium compounds containing pyrene and chrysene were found to be approximately 4 orders of magnitude larger, respectively, than those for the corresponding Fe complexes. This is believed to be an effect of the difference in the sizes of the two metals and their subsequent susceptibility toward nucleophilic attack. The temperature and concentration dependence of the rate constant k was studied for the reaction of [(η5-C5(CH3)5)Ru(η6-anthracene)]+ with CH3CN. The ΔS* of-13.3 (9) eu and ΔH* = +14.9 (3) kcal/mol confirm the associative nature of the displacement reaction. The straight-line plot obtained for Kobsd vs. [CH3CN] is consistent with a rate equation of the general form rate = k[M] [CH3CN] where [M] is the concentration of the metal complex. Two mechanisms consistent with the data are proposed and discussed; one includes a preequilibrium between a η6-arene metal complex and a η4-arene species, while the other involves direct nucleophilic attack on the metal center of the η6 complex.