Iron II complexes with nitrogen ligands

Atypical results have been found in ferrous complexes containing the [Fe(NH3)6] + cation [69]. [Fe(NH3)6][Fe2(CO)g] and [Fe(NH3)6]Cl2 both 

Independent work on [Fe(NH3)6]Cl2 has found that the appearance of a doublet spectrum at low temperatures is accompanied by hysteresis effects 

A number of spectra have been measured for pyridine complexes of the general type Fe(py)4X2 where X = Cl, Br, I, NCS, or NCO [71-74]. The quadrupole splittings, chemical isomer shifts, and magnetic moments jx) are listed in Table 6.4. The shift is generally lower than in complexes with coordination to oxygen by about 0-1-0-2 mm s .  

Yellow and violet forms of Fe(py)4(NCS)2 are known and were originally 

If all the pyridine complexes in Table 6.4 are considered to have a tram-geometry then the quadrupole splitting will provide a measure of the elongation or compression along this axis. It is interesting to note that in the pairs Fe(py)4l2/Fe(py)j2-2py and Fe(py)4(NCO)2/Fe(py)4(NCO)2.2py where there is no change in the primary coordination sphere there is little change in the Mossbauer parameters. 

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Martin et al. have developed a unique series of capped tris(l,2-di-aminoethane) cages which can encapsulate divalent transition metal ions in a near octahedral geometry (28). The iron(II) complex with the ligand (NH2)2sar turns out to be a crossover system in solution [34], but the solid triflate salt is low-spin [43]. This is the only Fe(II) crossover system having 6 identical aliphatic nitrogen donors. 

In the same way that iron (II) has a particularly strong affinity for nitrogen-donor ligands, so does iron(III) for oxygen donors. Most iron(II) complexes with simple oxygen... 

These compounds are the first examples for the stabilization of the LS state of iron(II) with saturated nitrogen as the most abundant donors this has been ascribed by the authors to the enhanced ligand field strength due to the constrictive effect of the mechanically confining in-plane macrocyclic ligand . Six-coordinate iron(II) complexes are formed with relatively weak axial ligands such as X = CH3C002 >... 

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