Spectroscopic and kinetic studies of the reactions below (X = H, 2-Me, 3-Me, 4-Me, 2-CI, 3-CI, 4-CI, 4-OMe, or 4-NO2) in CH,CN have provided quantitative information on the importance of basicity and steric factors in controlling amine nucleophilicity towards co-ordinated x-hydrocarbons. Similar reactions with cations [Fe(l -5-q-C6H,)-(CO),]+ (la) and [Fe(l-5--q-C,H9)(C0),]+ ( l c ) have also been investigated. For the reactions of 4-methoxyand 4-methyl-aniline with (1 b), which proceed to completion, kobs. = k,[amine].However, for the equilibrium X X X x C ~H & N H ~+ + reactions of the other less basic amines with cations (la)-(lc) the two-term expression kobe. = k,[amine] + k_,[H+]/([H+] + K2Ka) is indicated. The general rate trend C,H, > 2-MeOC6H, > C,H, and the low AH:, and large negative AS:, values are consistent with direct addition to the dienyl rings. For attack by non-sterically crowded anilines on (1 b), a Bronsted plot of log k, versus pKa of the amine conjugate acid (in H,O) has a high slope of 1 .O, indicating a verystrong dependence of kl on amine basicity. A Hammett plot for this reaction gives a slope of -2.7, indicating significant bond formation and build-up of positive charge on the aniline nitrogen atom in the transition state. The steric retardation of k, caused by blocking substituents in the 2-position of the anilines is considerably smaller than that previously found for the related additions of pyridines to cations (la)-(1 c).