The Challenge of Being Straight:  Explaining the Linearity of a Low-Spin {FeNO}7 Unit in a Tropocoronand Complex

We have carried out a density functional theory study of the S = 1/2 {FeNO}7 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 linear NO in an equatorial position and a very short Fe−NNO 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}7 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 dz2-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 dx2-z2 in character, with the Fe−NNO vector being identified as the z 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−NNO distance.