Quantification of the β-stabilizing effect of the dicarbonyl(η5-cyclopentadienyl)iron group

Kinetic investigations of the reactions of (prop-2-enyl)dicarbonyl (cyclopentadienyl) iron complexes 1 with benzhydrylium ions 3, and of dicarbonyl(cyclopentadienyl)[(1,2-eta)propene]iron (II) tetrafluoroborate (9(.)BF(4)) with pi-nucleophiles have been performed to elucidate the magnitude of the beta-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 pi-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 pi-nucleophiles. Replacement of one beta-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 = -11.2).