Partial isomer shifts

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. 

Recently, in a study of a large number of organo-iron compounds 120), it has been found possible to treat the isomer shift as being due to a sum of partial isomer shifts 8,. from individual ligands which for 7r-cyclopentadienyl derivatives could be correlated with proton NMR chemical shifts in the same compounds. The Mossbauer spectra of two cases of theoretical interest are worthy of mention. First, the spectra of both the mono- and binuclear iron tricarbonyl derivatives of cyclooctatetraene 115) show similar 8 and A values. The small 8 values are consistent with zero oxidation state for the iron atom and essentially complete covalent bonding between the iron and the tt electrons of the ring, so that, at least for the mononuclear... 

When all six ligands are identical, derivation of the partial isomer shift is straightforward, e.g. 

Table 2.10. Partial isomer shift values for low spin iron(II)"...

Bonding in transition metal complexes 4.1 Deductions from partial isomer shift and partial quadruple... 

Both parameters are sensitive to the donor-acceptor characteristics of a ligand, but they respond differently. The partial isomer shift represents the contribution to s electron density at the nucleus, which is increased both by o-donation, and by Ti-withdrawal of d electron density. The partial quadrupole splitting represents charge placed in the d orbitals, and becomes more negative as a-donation increases, but more positive with increase in metal to ligand rr-bonding. This means that... 

Fig. 2.15. Correlation diagrams (partial quadrupole splitting against partial isomer shift) for (a) iridium(III), and (b) ruthenium(II).

Fig. 2.16. Correlation diagram for low spin iron(II). Note that the partial isomer shift axis is reversed.

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