The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe (TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated) and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of ν C-O in toluene solution (1969-72 cm −1 ) but a large range of CO stretching frequencies in the solid-state cm −1 ). The large solid-state variation results from CO interactions in the solid-state as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-C-O and N Im -Fe-CO units. The values of ν C-O and the Fe-C and C-O bond distances are strongly correlated and provide a structural as well as a spectroscopic correlation of the π back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

The synthesis, characterization, and X-ray structures of three low-spin (nitrosyl)iron(II) tetraarylporphyrinates, [Fe(TpXPP)(NO)(1-MeIm)], where X = F (in a triclinic and a monoclinic form) and OCH 3 are reported. All three molecules, at 100 K, have a single orientation of NO. These structures are the first examples of ordered NO's in [Fe(Porph)(NO)(1-MeIm)] complexes. The three new derivatives have similar structural features including a previously unnoted "bowing" of the N NO -Fe-N Im angle caused by a concerted tilting of the axial Fe-N NO and Fe-N Im bonds. Structural features such as the displacement of Fe out of the mean porphyrin plane toward NO, tilting of the Fe-N NO bond off the heme normal, and the asymmetry of the Fe-N por bonds further strengthen and confirm observations from earlier studies. The [Fe(TpXPP)(NO)(1-MeIm)] complexes were also studied at temperatures between 125-350 K to investigate temperature-dependent variations and trends in the coordination group geometry. At varying temperatures (above 150 K), all three derivatives display a second orientation of the NO ligand. The population and depopulation of this second orientation is thermally driven, with no apparent hysteresis. Crystal packing appears to be the significant feature in defining the order/disorder of the NO ligand. The length of the bond trans to NO, Fe-N Im , was also found to be sensitive to temperature variation. The Fe-N Im bond length increases with increased temperature, whereas no other bonds change appreciably. The T-dependent Fe-N Im bond length change and cell volume changes are consistent with a "soft" Fe-N Im bond. Variable temperature measurements show that the N-O stretching frequency changes with the Fe-N Im bond length. Temperature-dependent changes in the Fe-N Im bond length and N-O stretching frequency were also found to be completely reversible with no apparent hysteresis.