Toward binary nitrosyls: Distinctly bent Fe-N-O linkages in base-stabilized Fe(NO)(3)(+) complexes

Air- and moisture-sensitive Fe(NO)(3)(eta(1)-PF6) (1) may be conveniently prepared by treating Fe(NO)(3)Cl with 1 equiv of [Ag][PF6] in CH2Cl2 or by reacting [NO][PF6] with excess iron filings in MeNO2. Complex 1 is thermally sensitive both as a solid and in solutions, and is best handled below -20 degreesC. To isolate 1 reproducibly from MeNO2 solutions it is necessary to remove all traces of propionitrile, which often occurs as an impurity in MeNO2, because it reacts with Lewis-acidic 1 to form [Fe(NO)(3)(EtCN)][PF6] (2). If trace H2O is present during the synthesis of 1, some of the PF6- is converted to PO2F2-, which is sufficiently Lewis basic that it captures two Fe(NO)(3)(+) fragments and forms [(ON)(3)Fe(mu-PO2F2)Fe(NO)(3)]-[PF6] (3). Finally, Fe(NO)(3)(eta(1)-BF4) (4) can be obtained as a green microcrystalline powder by employing the same synthetic methodologies used to prepare 1. The new complexes 1-4 have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 2, 3, and 4 and their parent compound, Fe(NO)(3)Cl, have been established by X-ray diffraction methods. The iron centers in the Fe(NO)(3) fragments in all these structures exhibit approximately tetrahedral coordination geometries, and the Fe-N-O linkages are distinctly nonlinear with bond angles in the range of 159 to 169degrees. DFT calculations on Fe(NO)(3)(eta(1)-BF4) (4) confirm that its bent Fe-N-O links have an electronic origin and need not be attributed to other factors, such as packing forces in the crystal. Interestingly, the bending of the NO ligands results in an increase in the energy of the HOMO, relative to the linear case, but at the same time causes a decrease in energy of the HOMO-1 and the HOMO-2 molecular orbitals. This more than compensates for the higher energy of the HOMO, resulting in a lower energy structure.