Mossbauer effect partial isomer shift

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

The charge density at the nucleus is mainly determined by s electrons and only partially by p-elec-trons. The main effect of the p electrons and d electrons and any other electron shells that do not contribute directly to the electron density ip(0) p, is to shield the s electrons. The determination of the scale factor (r -rl) in Equation [9] is called the isomer shift calibration. The interpretation of isomer shifts in Mossbauer spectra involves the correlation of a given A/ (0)=l /(0)l -l /(0)lg value (the electron density difference) with the known electronic structure of the Mossbauer atom or the change of the... 

The electronic density at the nucleus may be separated into contributions due to inner-core filled orbitals and to partially filled valence orbitals, and thus the isomer shift may be expected to be sensitive to the specific nature of the chemical bonds involving the Mossbauer-active species and adjacent atoms in the molecule or lattice. Furthermore, the value of 8 may be modified not only by varying the 5-orbital occupation but also by a shielding effect associated with orbitals of other types. The removal of a d-electron from an iron ion, for instance, will lead to a contraction of the occupied s-orbitals and thus to a shift of 8 in the negative direction. A correlation between the isomer shift and oxidation states, which may be useful in the analysis of certain spectral features associated with iron in molecules and other materials, is shown in Figure 3. 

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