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

[Image: see text] NO is well-known for its trans effect. 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 axial trans Fe–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 the trans Fe–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 observed trans-bond shortening in some NO adducts of ferric porphyrins. We show that the strong σ antibonding interaction of d(z(2)) and 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 net trans-bond shortening in the {FeNO}(6) but not the {FeNO}(7) derivatives.