Description: The knowledge of accurate bond strengths is a fundamental basis for a proper analysis of chemical reaction mechanisms. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe–O and Fe–S bond energies of (meta-substituted phenoxy)dicarbonyl(η 5 -cyclopentadienyl) iron [ m -G-C 6 H 4 OFp ( )] and (meta-substituted benzenethiolato)dicarbonyl(η 5 -cyclopentadienyl) iron [ m -G-C 6 H 4 SFp ( )] complexes. In this study, Fp is (η 5 -C 5 H 5 )Fe(CO) 2 , and G is NO 2 , CN, COMe, CO 2 Me, CF 3 , Br, Cl, F, H, Me, MeO, and NMe 2 . The results show that Tao–Perdew–Staroverov–Scuseria and Becke's power-series ansatz from 1997 with dispersion corrections functionals can provide the best price/performance ratio and accurate predictions of Δ H het (Fe–O)'s and Δ H het (Fe–S)'s. The excellent linear free energy relations [ r  = 1.00 (g, 1e), 1.00 (g, 2b)] among the ΔΔ H het (Fe–O)'s and δΔ G 0 of O H bonds of m -G-C 6 H 4 OH or ΔΔ H het (Fe–S)'s and Δp K a 's of S H bonds of m -G-C 6 H 4 SH imply that the governing structural factors for these bond scissions are similar. And, the linear correlations [ r  = −0.97 (g, 1 g), −0.97 (g, 2 h)] among the ΔΔ H het (Fe–O)'s or ΔΔ H het (Fe–S)'s and the substituent σ m constants show that these correlations are in accordance with Hammett linear free energy relationships. The inductive effects of these substituents and the basis set effects influence the accuracy of Δ H het (Fe–O)'s or Δ H het (Fe–S)'s. The ΔΔ H het (Fe–O)'s(g) (1) and ΔΔ H het (Fe–S)'s(g)(2) follow the capto-dative Principle. The substituent effects on the Fe–O bonds are much stronger than those on the less polar Fe–S bonds. Insight from this work may help the design of more effective catalytic processes. Copyright © 2016 John Wiley & Sons, Ltd. The heterolytic Fe–O and Fe–S bond dissociation energies of 12 meta-substituted phenoxydicarbonyl(η 5 -cyclopentadienyl) iron and 12 (meta-substituted benzenethiolato)dicarbonyl(η 5 -cyclopentadienyl) iron were studied using density functional theory methods with large basis sets. The results show that Tao–Perdew–Staroverov–Scuseria and Becke's power-series ansatz from 1997 with dispersion corrections functionals can provide the best price/performance ratio and accurate predictions of Δ H het (Fe–O)′s and Δ H het (Fe–S)'s. Density functional theory methods satisfactorily predict the substituent effects on Δ H het (Fe–O)'s and Δ H het (Fe–S)'s. The ΔΔ H het (Fe–O)'s(g)(1) and ΔΔ H het (Fe–S)'s(g)(2) follow the capto-dative principle. The substituent effects on the Fe–O bonds are much stronger than those on the less polar Fe–S bonds.