Iron-57 compounds chemical isomer shift

An initial report that the crystal structure of [Co(NH3)e]-[FeCle] is isomorphous with that of ]Co(NH3)6][TlCl6] has been shown to be incorrect. However, the Mossbauer chemical isomer shifts for [Co(en)3][FeCl6] and [Co(NH3)e][FeCl6] of 0.60 + 0.10 and 0.59 0.10 mm./second, respectively, agree with the values found for other hexacoordinate iron(III) compounds and differ considerably from the value of 0.40 mm./... 

The second-order Doppler shift (see Chapter 3.2) is usually not negligible in iron compounds, and because it is a function of the lattice dynamics of the compound, which are rarely understood, it is difficult to eliminate prior to making comparisons of j-electron densities. Consequently there is usually a limit to the significance of small differences in chemical isomer shift in related compounds and care should be taken not to over-analyse data in terms of very small changes in electron density. 

Spectra of several tris(hydroxamato) iron(III) complexes such as tris(sali-cylhydroxamato) iron(III) trihydrate show chemical isomer shifts typical of high-spin iron(III) compounds, but gross relaxation broadening complicates any attempt at detailed interpretation [135]. 

Compounds (1-5) are all very similar with one unpaired electron per iron atom, large quadrupole splittings, and a chemical isomer shift compatible with a low-spin Fe(lII) S = 1 configuration. All may therefore be presumed to have the same dimeric structure IV. The quadrupole splittings range from... 

Compounds (6-8) contain the basic structural unit [Fe(py) —S2 2] they have three unpaired electrons and are probably penta-coordinate Fe(IIl) S = i compounds (structure V) similar to the bis-(N,N -dithiocarbamato)-iron(III) halides discussed in the preceding section. Both series show an approximately systematic variation in A with change in the ligand which is not matched by a corresponding variation in the chemical isomer shift, so that it seems unlikely that large changes in delocalisation are occurring. The very small temperature dependence of A in the S = i complexes makes it difficult to determine the electronic level separations. 

The compound now formulated as [(OC)3FePMe2Ph]3 was found to be a trinuclear species rather than di- or tetra-nuclear from the similarity of its Mossbauer spectrum to Fe3(CO)i2 [5]. All three iron atoms have shown a change in chemical isomer shift, and this leads one to suggest that one terminal carbonyl group on each iron atom has been substituted. This has since been confirmed by an X-ray crystal structure analysis [28]. 

The reaction of peroxidase with HjOa produces at least three intermediates (signified by I, II, and III). All three give a sharp quadrupole splitting with only a small temperature dependence (Table 13.3) [20]. Compounds I and II give an unusually low chemical isomer shift and are indistinguishable. It has been proposed that II is an iron(IV) compound with two unpaired electrons, and this would be compatible with the Mossbauer results. Compound III appears to be similar to oxyhaemoglobin and may be structurally analogous. 

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... 

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