Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday Kinetic investigations of the reactions of (prop-2-enyl)dicarbonyl(cyclopentadienyl)iron complexes 1 with benzhydrylium ions 3, and of dicarbonyl(cyclopentadienyl)[(1,2-h)propene]iron(II) tetrafluoroborate (9 ¥ BF 4 ) with p-nucleophiles have been performed to elucidate the magnitude of the b-effect of the [(CO) 2 FeCp] group (Fp group). Introduction of the Fp group into the allylic position of propene and 2-methylpropene increases the nucleophilicity of the p-bonds by nine and six orders of magnitude, respectively, with the result that the allyl-Fp complexes 1a (N 6.78) and 1b (N 8.45) are among the strongest neutral p-nucleophiles. Replacement of one b-H-atom in the isopropyl cation by the Fp group reduces the electrophilicity by more than 20 orders of magnitude, so that 9 ranks among the weakest cationic C-electrophiles (E À 11.2).