57Fe Mossbauer spectra

The 57Fe Mossbauer spectra for [FexCo x(acpa)2]BPh4 (x=0.035 and 0.074) show that the transition has been displaced to a higher temperature than that for the pure Fe(III) compound [156]. This may again be related to 

Fig. 11 57Fe Mossbauer spectra of (bt, S) recorded at 4.2 K in zero-field before irradiation (a), immediately after irradiation (b), 6 days (c) and 11 days (d) after irradiation. Mossbauer subspectra correspond to HS species (grey), LS species (dark grey)

Fig. 19. 57Fe Mossbauer spectra of [Fe(2-pic)3]Cl2 EtOH at various temperatures84

It has been postulated that the origin of the two different high-spin Fe(II) doublets observed in the 57Fe Mossbauer spectra may be that a small fraction (about 6%) of the Fe(II) ions experience a different local environment, most likely in the distribution of the non-coordinating solvent and anion molecules, from that of the majority of the high-spin Fe(II) ions. 

The inter-conversion of the spin states in many instances is so rapid that the separate contributions to the 57Fe Mossbauer spectra are not resolved. Thus this technique, which has proved so diagnostic in iron(II) systems, is frequently less suited to the derivation of spin transition curves for iron(III). A further corollary of the faster spin state inter-conversion is the rarity of the LIESST effect among iron(III) systems, in contrast to its ubiquitous occurrence in iron(II). 

Reaction of FeCo2(CO)9S with a series of phosphines (31, 133) and isocyanides (126) yielded mono-, di-, and trisubstituted derivatives, Eqs. (77) and (78). 57Fe-Mossbauer spectra of the phosphine-substituted derivatives indicated that substitution at cobalt occurs prior to substitution at iron (31). Unfortunately, no crystallographic evidence has been obtained for any of these derivatives, and the precise stereochemistry has not been resolved, even with the aid of l3C-NMR spectra (9). The problem is compounded with the isocyanide ligands since several isomers of the trisubstituted derivatives are formed. 

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