Protoporphyrin IX iron(II) revisited. Overview of the Mössbauer spectroscopic parameters of low spin porphyrin iron(II) complexes Part II

Mössbauer parameters of low spin six coordinate [Fe(II)(Por)L ₂ ] and [Fe(II)(Por)LX] complexes (where Por is a synthetic porphyrin; L is OH ⁻ , H ₂ O, a nitrogenous base, CCl ₂ , CO or CN ⁻ and X is a different ligand to L) are discussed. Our point charge calculation approach is extended to investigate how the axial ligands and the four porphyrinato-N atoms generate the observed quadrupole splittings (ΔE _Q ). Partial quadrupole splitting (p.q.s.) and partial centre shift (p.c.s.) values are derived and discussed for all the axial ligands studied herein. With these we were able to address several porphyrin problems reported by others. It is shown that when the D _4h symmetry breaks down due to porphyrin ring ruffling, the point charge calculations no longer apply. It is also shown that this happens for many synthetic porphyrins but not for the important [Fe(II)(PPIX)] moiety, (which is ubiquitous in nature). In all the haem proteins containing [Fe(II)(PPIX)] moieties the haem is planar. The calculations indicate that if H ₂ O were bound to the [Fe(II(PPIX))] present in deoxyheamoglobin or deoxymyoglobin, then these Fe(II) atoms would be low spin. In contrast for synthetic and natural haem proteins containing haem b (where the haem is connected to the protein backbone via two cysteine residues) the haem plane is ruffled (probably because of the methionine linkages transferring force and controlling the stereo chemistry of the haem plane, and thereby its chemistry). For the [Fe(II)(TPP)(CN) ₂ ] and [Fe(II)(TPP)(1-MeIm)(CN)] the calculated CN ⁻ ligand p.q.s values are suspect and probably show that the lattice charge is affecting them.