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 Fe-C bond dissociation energies [i.e., DH homo (Fe-C)s] of 12 para-substituted benzyldicarbonyl(h 5 -cyclopentadienyl)iron, p-G-C 6 H 4 CH 2 Fp [1,G ¼ NO 2 , CN, COMe, CO 2 Me, CF 3 , Br, Cl, F, H, Me, MeO, NMe 2 ; Fp¼ (h 5 -C 5 H 5 )(CO) 2 Fe] and 12 para-substituted a-cyanobenzyldicarbonyl (h 5 -cyclopentadienyl)iron, p-G-PANFp [2,PAN ¼ C 6 H 4 CH(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 DH homo (Fe-C)s. The B3LYP method satisfactorily predicts the a and remote substituent effects on DH homo (Fe-C)s [DDH homo (Fe-C)s]. The fair correlations [r ¼ 0.97 (g, 1), 0.99(g, 2)] of DDH homo (Fe-C)s of series 1 and 2 with the substituent s R p constants imply that the para substituent effects on DH homo (Fe-C)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 DDH homo (Fe-C)s(1) varies significantly but also the direction changes from S-pattern to O-pattern. DDH homo (Fe-C)s(2) were found to conform to the Capto-dative Principle. The detailed knowledge of the factors that determine the Fp-C bond strengths would greatly aid in understanding reactivity patterns in many processes.