Understanding the Unusually Straight: A Search For MO Insights into Linear {FeNO}7 Units

Ferrous-nitrosyl {FeNO}(7) complexes, whether S = 1/2 or 3/2, generally exhibit bent FeNO angles of around 140-145 degrees. There are, however, a handful of exceptions, which are characterized by linear or quasi-linear FeNO units. Presented herein is a relatively comprehensive DFT-based MO analysis of these unusual {FeNO)(7) complexes. DFT-derived FeNO bending potentials indicate that the unusual, experimentally observed quasi-linear geometries indeed correspond to minimum-energy structures on the potential energy surfaces of the isolated molecules/ions. Walsh diagram analyses support our earlier suggestion that the linearity of the {FeNO}(7) units in question is most commonly attributable metal d(sigma)-p(sigma) mixing resulting from the lack of a ligand trans to the NO. Importantly, this effect explains the linearity of both S = 1/2 {FeNO}(7) complexes such as [Fe(CN)(4)(NO)](2-) and Fe(dtc-Me-2)(7)(NO) (dtc-Me-2 = NN-dimethyldithiocarbamate) and S = 3/2 complexes such as [Fe(S'Bu)(3)(NO)](-). However, Roussin's black salt anion, (Fe-4(mu-S)(3)(NO)(7)](-), which also contains a linear {FeNO}(7) unit, entails additional, special metal-ligand orbital interactions. The well-known brown-ring complex [Fe(H2O)(5)(NO)](2+) also contains a linear {FeNO}(7) unit; the linearity in this case is attributable to the weakness of the trans water ligand.