Five-Coordinate FeIIINO and FeIICO Porphyrinates:  Where Are the Electrons and Why Does It Matter?

Recent years have seen dramatic growth in our understanding of the biological roles of nitric oxide (NO). Yet, the fundamental underpinnings of its reactivities with transition metal centers in proteins and enzymes, the stabilities of their structures, and the relationships between structure and reactivity remains, to a significant extent, elusive. This is especially true for the so-called ferric heme nitrosyls ({FeNO}6 in the Enemark−Feltham scheme). The Fe−CO and C−O bond strengths in the isoelectronic ferrous carbonyl complexes are widely recognized to be inversely correlated and sensitive to structural, environmental, and electronic factors. On the other hand, the Fe−NO and N−O bonds in {FeNO}6 heme complexes exhibit seemingly inconsistent behavior in response to varying structure and environment. This report contains resonance Raman and density functional theory results that suggest a new model for FeNO bonding in five-coordinate {FeNO}6 complexes. On the basis of resonance Raman and FTIR data, a direct correlation between the νFe - NO and νN - O frequencies of [Fe(OEP)NO](ClO4) and [Fe(OEP)NO](ClO4)·CHCl3 (two crystal forms of the same complex) has been established. Density functional theory calculations show that the relationship between Fe−NO and N−O bond strengths is responsive to FeNO electron density in three molecular orbitals. The highest energy orbital of the three is σ-antibonding with respect to the entire FeNO unit. The other two comprise a lower-energy, degenerate, or nearly degenerate pair that is π-bonding with respect to Fe−NO and π-antibonding with respect to N−O. The relative sensitivities of the electron density distributions in these orbitals are shown to be consistent with all published indicators of Fe−N−O bond strengths and angles, including the examples reported here.