The Challenge of Being Straight:  Explaining the Linearity of a Low-Spin {FeNO}<sup>7</sup> Unit in a Tropocoronand Complex Espen Tangen Jeanet Conradie Abhik Ghosh 10.1021/ic050781a.s001 https://acs.figshare.com/articles/journal_contribution/The_Challenge_of_Being_Straight_Explaining_the_Linearity_of_a_Low_Spin_FeNO_sup_7_sup_Unit_in_a_Tropocoronand_Complex/3254599 We have carried out a density functional theory study of the <i>S</i> = <sup>1</sup>/<sub>2</sub> {FeNO}<sup>7</sup> tropocoronand complex, Fe(5,5-TC)NO, as well as of some simplified models of this compound. The calculations accurately reproduce the experimentally observed trigonal-bipyramidal geometry of this complex, featuring a <i>linear</i> NO in an equatorial position and a very short Fe−N<sub>NO</sub> distance. Despite these unique structural features, the qualitative features of the bonding turn out to be rather similar for Fe(5,5-TC)NO and {FeNO}<sup>7</sup> porphyrins. Thus, there is a close correspondence between the molecular orbitals (MOs) in the two cases. However, there is a critical, if somewhat subtle, difference in the nature of the singly occupied MOs (SOMOs) between the two. For square-pyramidal heme−NO complexes, the SOMO is primarily Fe d<i><sub>z</sub></i><sup><sub>2</sub></sup>-based, which favors σ-bonding interactions with an NO π* orbital, and hence a bent FeNO unit. However, for trigonal-bipyramidal Fe(5,5-TC)(NO), the SOMO is best described as primarily Fe d<i><sub>x</sub></i><sup><sub>2</sub></sup><sub>-</sub><i><sub>z</sub></i><sup><sub>2</sub></sup> in character, with the Fe−N<sub>NO</sub> vector being identified as the <i>z</i> direction. Apparently, such a d orbital is less adept at σ bonding with NO and, as such, π bonding dominates the Fe−NO interaction, leading to an essentially linear FeNO unit and a short Fe−N<sub>NO</sub> distance. 2005-11-28 00:00:00 SOMO FeNO unit z 2 MO