High spin iron II coordination complexes

In the absence of spin-orbit coupling and in true octahedral or tetrahedral geometry, the first five 3d electrons of the partial electronic configuration, or e t2 respectively, will lead to a spherically 

The various iron(ll) complexes of pyridine provide a range of related compounds which illustrate how the Mossbauer parameters respond to changes in coordination environment and coordination geometry as well as structural phase transformations. 

The iron(Il) halide and pseudohalide coordination complexes formed with pyridine may be divided into two types. The first type consists of complexes with the general stoichiometry, Fe(pyridine)4X2, where X represents the halides and pseudohalides listed in Tables 3.4 and 3.5. They are found to be high spin monomeric tetragonally distorted trans octahedral complexes and show paramagnetic behaviour down to around 

Reaction Solvent X No. Environ- ment Coordination Approximate geometry 

xHjO+4 pyridine - Fe(pyridine)4X2 pyridine Cl,Br,I 6 N4X2 octahedral monomer s 

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Nasri H, Ellison MK et al (2006) Electronic, magnetic, and structural characterization of the five-coordinate, high-spin iron (II) nitrato complex [Fe(Tpivpp)(N03)] . Inorg Chem 45 5284-5290... 

Iron(II) complexes of general formula [FeLA2] A = Cl, Br, I, NO3, NCS or NCSe have been formed [145] with both 2-formylquinohne thiosemicarbazone, 20, and 1-formylisoquinoline thiosemicarbazone, 21. All are characterized as five-coordinate, high spin iron(II) complexes and the electronic spectra are assigned consistent with distorted trigonal bipyramidal structures. 

Measurements of the proximal histidine-iron stretching frequency by Resonance Raman spectroscopy revealed that this bond is very weak in relation to other heme protein systems (vFe.His = 204 cm-1) (130). Formation of the sGC-NO complex labilizes this ligand resulting in the formation of a 5-coordinate high spin iron(II) complex, and the conformational change responsible for the several hundred-fold increase in catalytic activity (126,129,130). 

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