Electric field gradient iron ions

The Mossbauer effect is sensitive to the oxidation and spin state of iron and the environment around the iron nucleus therefore different chemical species yield different Mossbauer spectra. Furthermore all spin states and oxidation states of iron are accessible to the technique. There are three main components of a Mossbauer spectrum. The isomer shift arises from the electron density at the 57Fe nucleus the quadrupole splitting results from the electric field gradient produced by electrons and ions around the 57Fe nucleus, and the nuclear Zeeman splitting is sensitive to the spin state and magnetic coupling of the iron. 

Low-spin iron (II) ions are d with no unpaired electrons and are diamagnetic. The t2g subshell is fully occupied, which leads to a symmetric charge distribution around the iron nucleus. Therefore, the electronic contribution to the electric field gradient should diminish and as a consequence low-spin iron(II) complexes are expected to have a very small or even undetectable quadrupole splitting. However, this is not always the case. Note that some low-spin ferrous heme proteins as well as heme model complexes have A g > Imms L For example, for the heme model complex [TMPFe(II)(l,2-Me2lm)2], A g = 1.73 mms has been reported. 

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