With the reactant serving as the TDI, the turnover-determining transition state (TDTS) is the one that maximizes the free energy difference relative to the TDI. When spin crossover is allowed, TS1 is the TDTS for Fe and Ru variants of complexes D and F, except for complex D with Fe where the HAT transition state (TS2) is the TDTS. When spin crossover is disallowed, both Fe and Ru variants for complex D have TS2 as the TDTS while complex F has TS1 as the TDTS (Figure and Supporting Information Table ). As expected, Ru catalysts yield optimal energetic spans when spin crossover is allowed (ca. 24 kcal/mol for D and 15.1 kcal/mol for F) while for Fe catalysts, allowing or restricting spin crossover makes no difference on the resulting energetic spans (25 kcal/mol for D and 21 kcal/mol for F) since they proceed via spin-allowed C-H hydroxylation pathways (Figure and Supporting Information Table ). We note that one of the Fe catalysts has a LS ground state at the isopropanol-bound intermediate while the metalhydroxo intermediate and product have IS ground states (Figure ). The IS isopropanol-bound intermediate, however, is only 1 kcal/mol higher in energy than the LS ground state, such that the reaction may proceed entirely on the IS state with no spin crossover to the LS state at this intermediate. Nevertheless, while the TDTS and TDI can be sensitive to the functional choice, 120 the energetic spans of the Fe and Ru catalysts are still qualitatively quite comparable. With Fe catalysts having similar energetic spans to the experimentally proven Ru catalysts while exhibiting spin-allowed pathways, we propose that Fe catalysts provide a comparatively better handle for tuning C-H activation reactivity over Ru catalysts. The experimental activity of Ru catalysts in the functionalization of C-H bonds in polyolefins is attributed to their compatibility with the reaction conditions including parameters like temperature, concentrations of reagents, and solvent type. To harness the potential of Fe catalysts for this application, complementary screening measures can be employed that seek optimal reaction conditions by adjusting these parameters.