What can isomer shift data tell us

The isomer shift gives a measure of the electron density on the Fe centre, and isomer shift values can be used to determine the oxidation state of the Fe atom. Similarly, in Au Mossbauer spectroscopy, isomer shifts can be used to distinguish between Au(I) and Au(III). Three specific examples are chosen here from iron chemistry. 

The cation [Fe(NH3)5(NO)] has presented chemists with an ambiguity in terms of the description of the bonding which has, in some instances, been described in terms of an [NO] unit bound to an Fe(I) centre. Results of Fe Mossbauer spectroscopy have revealed that the correct description is that of an [NO] ligand bound to an Fe(III) centre. 

The formal oxidation states of the iron centres in [Fe(CN)6] and [Fe(CN)g] are +2 and +3 however, the closeness of the isomer shifts for these species suggests that the actual oxidation states are similar and this may be interpreted in terms of the extra electron in [Fe(CN)g] being delocalized on the cyano ligands rather than the iron centre. 

The following terms were introduced in this chapter. Do you know what they mean  

Housecroft and E.C. Constable (2002) Chemistry, Prentice Flail, Harlow - Chapter 14 covers first order reaction kinetics with worked examples, and includes mathematical background for the integration of rate equations. 

The isomer shift gives a measure of the electron density on the Fe centre, and isomer shift values can be used to 

Differences in isomer shifts can be used to distinguish different iron environments in the same molecule the existence of two signals in the Mossbauer spectrum of Fe3(CO)i2 provided the first evidence for the presence of two types of iron atom in the solid state stmcture (Fig. 4.29), a fact that has been confirmed by X-ray diffraction methods. 

Nucleus observed Natural abundance / % Ground spin state Excited spin state Radioisotope source 

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