In general, both stoichiometric and catalytic reactions of organometallic complexes involve breaking and forming metal–ligand bonds. Therefore, an evaluation of the thermodynamics of such reactions requires the knowledge of metal–ligand bond energies (BDEs). The homolytic FeC bond dissociation energies [i.e., ΔHhomo(FeC)s] of 12 para‐substituted benzyldicarbonyl(η5‐cyclopentadienyl)iron, p‐G‐C6H4CH2Fp [1,G = NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO, NMe2; Fp = (η5‐C5H5)(CO)2Fe] and 12 para‐substituted α‐cyanobenzyldicarbonyl (η5‐cyclopentadienyl)iron, p‐G‐PANFp [2,PAN = C6H4CH(CN)] were studied using Hartree–Fock (HF) and density functional theory (DFT) methods with large basis sets. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔHhomo(FeC)s. The B3LYP method satisfactorily predicts the α and remote substituent effects on ΔHhomo(FeC)s [ΔΔHhomo(FeC)s]. The fair correlations [r = 0.97 (g, 1), 0.99(g, 2)] of ΔΔHhomo(FeC)s of series 1 and 2 with the substituent σ constants imply that the para substituent effects on ΔHhomo(FeC)s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. The molecule stabilization effects (MEs) causes that not only the magnitude of ΔΔHhomo(FeC)s(1) varies significantly but also the direction changes from S‐pattern to O‐pattern. ΔΔHhomo(FeC)s(2) were found to conform to the Capto‐dative Principle. The detailed knowledge of the factors that determine the FpC bond strengths would greatly aid in understanding reactivity patterns in many processes. Copyright © 2010 John Wiley & Sons, Ltd.