Structural diagrams for (a) FeTPP-Y, and (b) FeP-X porphyrins used, respectively, in experimental determination and computational modeling of backbonding trends. Y and X are the indicated substituents, listed in order of electron-donating to -withdrawing propensity (–X4 indicates that –X and –H substituents alternated around the ring). For 6-coordinate adducts, the experiments included 4-methyl imidazole (a histidine mimic), and the computations included imidazole as axial ligands.