Quantification of the β ‐Stabilizing Effect of the Dicarbonyl( η 5 ‐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‐ η )propene]iron(II) tetrafluoroborate ( 9 ⋅BF 4 ) with π ‐nucleophiles have been performed to elucidate the magnitude of the β ‐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 π ‐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 π ‐nucleophiles. Replacement of one β ‐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).