Nitrosyl-containing iron complexes (FeNO) have attracted many studies focusing on the the mechanisms of NO binding in heme and NO release by a photoactivated nitric oxide-releasing moiety complex. Fe-NO and Fe-CO bonds form by σ-donation to an unoccupied metal orbital and the bond is strengthened by π-back donation from a filled metal d-orbital to the unoccupied π* orbitals on the ligand. Unlike Fe-CO bonds which are linear, Fe-NO bonds are often bent, resulting from one extra electron residing in the NO π* orbitals. Hence fluctuations between linear and bent FeNO configurations occur and are correlated to the placement of the extra electron in either the partially occupied Fe 3d orbitals or in one of the unoccupied NO π* orbitals. The understanding of the complex bonding by means of theory and computation requires a multiconfigurational approach. In the present work, we consider fluctuations in geometric and electronic properties by sampling structures from an ab intio molecular dynamics (AIMD) simulation. Theoretical calculations (CASSCF and NEVPT2) reveal strong correlations between sampled properties and the principle Fe-N reaction coordinate. In addition to these correlations, we find that fluctuations of electronic character occur leading to a multireference description of the oxidation state of the iron metal center.

Multiconfigurational effects in iron-nitrosyl (FeNO) complexes

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