When being straight bends rules: A rationale for the linear FeNO unit in the low-spin square-pyramidal {FeNO}7 tetracyanonitrosylferrate(2−) anion

All low-spin S = 1/2 heme–NO complexes feature FeNO angles of about 140°. In contrast, the square–pyramidal [Fe(CN)4(NO)]2− complex features an exactly linear {FeNO}7 unit. We have sought here to determine a possible, simple molecular orbital (MO) rationale for these structural variations. A DFT-based (DFT = density functional theory) MO analysis shows that the linearity of the latter stems from the greater pyramidalization of the Fe center, relative to nitrosylheme, which results in significant differences in d orbital hybridization. Thus, the singly occupied molecular orbital (SOMO) of [Fe(CN)4(NO)]2− , while primarily Fe dz2dz2-based, also has a significant amount of 4pz character, which makes it less stereochemically active, accounting for the linearity of the FeNO unit.