Insights into the Observedtrans-Bond Length Variations upon NO Binding to Ferric and Ferrous Porphyrins with Neutral Axial Ligands

NO is well-known for itstranseffect. NO binding to ferrous hemes of the form (por)Fe(L) (L = neutral N-based ligand) to give the {FeNO}⁷(por)Fe(NO)(L) product results in a lengthening of the axialtransFe-L bond. In contrast, NO binding to the ferric center in [(por)Fe(L)]⁺to give the {FeNO}⁶[(por)Fe(NO)(L)]⁺product results in a shortening of thetransFe-L bond. NO binding to both ferrous and ferric centers involves the lowering of their spin states. Density functional theory (DFT) calculations were used to probe the experimentally observedtrans-bond shortening in some NO adducts of ferric porphyrins. We show that the strong σ antibonding interaction of d_z²and the axial (L) ligand p orbitals present in the Fe(II) systems is absent in the Fe(III) systems, as it is now in an unoccupied orbital. This feature, combined with a lowering of spin state upon NO binding, provides a rationale for the observed nettrans-bond shortening in the {FeNO}⁶but not the {FeNO}⁷derivatives.