The Electronic Configuration and Ligand Nature of Five Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study.

The five coordinate iron porphyrin carbene complexes [Fe(TPP)(CCl2)] (TPP = tetraphenylporphyrin), [Fe(TTP)(CCl2)] (TTP = tetratolylporphyrin) and [Fe(TFPP)(CPh2)] (TFPP = tetra(pentafluorophenyl)porphin), utilizing two types of carbene ligands (CCl2 and CPh2), have been investigated by single crystal X-ray, XANES (X-ray Absorption Near Edge Spectroscopy), Mössbauer, NMR and UV-vis spectros-copies. The XANES unambiguously suggested the iron(II) oxida-tion state of the complexes. The multi-temperature and high mag-netic field Mössbauer experiments, which show very large quadru-pole splittings (QS, ΔEQ), determined the S = 0 electronic configu-ration. More importantly, the combined structural and Mössbauer studies, especially the comparison with the low spin iron(II) por-phyrin complexes with strong diatomic ligands (CS, CO and CN(-)) revealed the covalent bond nature of the carbene ligands. A correla-tion between the iron isomer shifts (IS, δ) and the axial bond dis-tances is established for the first time for these donor carbon ligands (:C-R).