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 degrees. 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 d(alpha 2)-based, also has a significant amount of 4p(alpha) character. which makes it less stereochemically active, accounting for the linearity of the FeNO unit. (c) 2006 Published by Elsevier Inc.