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

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 z2 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.