The complexes [(benzene) 2M] 2+ (M = Fe, Ru, Os) react rapidlyand reversibly with tertiary phosphines to form cyclohexadienyl phosphonium ring adducts: ▪ Addition to both benzene rings does not occur even in the presence of excess phosphine. The room temperature 1H NMR spectra with P(n-Bu) 3 are static, but with PPh 3 exchange averaged spectra result. The rate and equilibrium constants with PPh 3 were obtained via variable temperature 31P and 1H NMR. In contrast to previously reported phosphine addition reactions, the electrophilic reactivity ( k 1) is very metal dependent: Fe > Ru > Os (390 : 7 : 1). The reverse rate constant ( k −1) is only weakly metal dependent, and the resulting linear free energy relationship (LFER) between K eq = k 1/ k −1 and k 1 implies considerable CPPh 3 bond formation in the transition state. It is suggested that metal → benzene π backbonding in [(C 6H 6) 2M] 2+ follows the order Ru, Os > Fe, and that this accounts for the greater thermodynamic and kinetic electrophilic properties of the iron complex. A mechanism for the phosphine addition reactions involving direct bimolecular attack on the arene is proposed. This and previous work allow the formulation of a rather extensive reactivity series for tertiary phosphine addition to coordinated cyclic π-hydrocarbons.