Experimental backbonding correlation of νMX and νXO for 5-coordinate Fe(II)CO, Fe(II)NO, Co(II)NO and Fe(II)O2 porphyrin adducts. For FeO2, literature values were gathered[33-35] for matrix-isolated species (TMP, OEP, TPP, Pc, TPP-F5) and for TMP in toluene solution. For FeCO [16], FeNO [15], CoNO [36], data were recorded in organic solvents (CH2Cl2, DMF, Bz) solution for a set of TPP-Y species (Fig. 3), where - Y are variably electron donating or withdrawing substituents. The indicated slopes are from least-squares fits to the data.