%0 Journal Article
%A Tangen, Espen
%A Conradie, Jeanet
%A Ghosh, Abhik
%D 2005
%T The Challenge of Being Straight: Explaining the Linearity of a Low-Spin
{FeNO}7 Unit in a Tropocoronand Complex
%U 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
%R 10.1021/ic050781a.s001
%2 https://acs.figshare.com/ndownloader/files/5089969
%K SOMO
%K FeNO unit
%K z 2
%K MO
%X 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.
%I ACS Publications