Iron complexes Mossbauer spectra

The spectroscopic properties of P clusters are unusual. In the dithi-onite-reduced MoFe protein all the Fe atoms of the P clusters are iron(II), indicating a [4Fe-4S] oxidation state, a level difficult to achieve with model complexes. Oxidation gives rise transiently to an EPR-observable (gav = 1.93) species, which then relaxes to give a very complex Mossbauer spectrum. 

Mackinawite, Fei+xS, with (x = 0.01-0.07) is a tetragonal iron sulfide with excess of iron. Morice et al. [153] reported a complex Mossbauer spectrum consisting of at least three sextets with hyperfine fields 29.8, 26.2 and 22.8 T and small quadrupole shifts of about 0.09, 0.06 and 0.09 mm/s respectively. On the other hand, only a singlet spectrum, even down to 4 K, has been observed by Vaughan and Ridout [154]. Probably the concentration of Co and Ni found to be present in the involved natural samples is decisive for the different magnetic behavior. 

The Debye temperature is usually high for metallic systems and low for metal-organic complexes. For metals with simple cubic lattices, for which the model was developed, is found in the range from 300 K to well above 10 K. The other extreme may be found for iron in proteins, which may yield d as low as 100-200 K. Figure 2.5a demonstrates how sharply/(T) drops with temperature for such systems. Since the intensity of a Mossbauer spectrum is proportional to the... 

Fig. 8.16 Fe Mossbauer spectra of [Fe2 (PMAT)2](BF4)4-DMF at selected temperatures. At 298 K, the only quadrupole doublet is characteristic of iron(II) in the HS state. SCO from HS to LS occurs at one Fe(II) site of the dinuclear complex at ca. 225 K. The second Fe(II) site remains in the HS state, but feels the spin state conversion of the neighboring atom by local distortions communicated through the rigid bridging ligand, giving rise to a new quadrupole doublet (dark gray), i.e., HS in [HS-LS], in the Mossbauer spectrum. The intensity ratio of the resonance signals of HS in [HS-LS] to that of LS (black) in [HS-LS] is close to 1 1 at all temperatures (from [32])...

The appearance of only one XPS peak for a mixed valence compound is consistent with a delocalized ground state (and excited state). Bifeirocenylene (II, III) picrate, whose structure is shown in Fig. 8, probably fits in this category. The Mossbauer spectrum of the complex indicates only one kind of iron atom, and the Fe 2p3,2 spectrum consists of only one peak with a weak shoulder at higher binding energy 29). It should be recognized, however, that even in the case of a localized system in which two XPS peaks are expected, if the chemical shift between the two peaks is less than the resolution of the spectrometer, only one peak will be observed. 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

If the spin state interconversion is faster than the excited nuclear state lifetime, that is x 10 7 second, then the observed spectrum is an average of the spectra of the two spin states. Until recently this condition had been observed only for iron(III) complexes with thiocar-bamate or selenocarbamate ligands—ferric dithiocarbamates (119), monothiocarbamates (98), or diselenocarbamates (42). Since 1982, however, there have been a number of reports of other iron(III) complexes which also display an averaged Mossbauer spectrum (56, 57, 108 111, 124, 153, 155). 

If some iron(III) complexes undergo rapid spin interconversion on the Mossbauer time scale, and some undergo slow interconversion, then it is inevitable that a few will interconvert, at some accessible temperature, at a rate which produces dynamic effects on the Mossbauer spectrum. Such examples have now been found (109, 111). Rate constants have been extracted from these spectra and are necessarily of the order of 106 -107 sec"1. The interpretation of the spectral lineshapes is complex (153, 154), however, and further work will be needed to establish the reliability of the rate data obtained from such spectra. 

CQ—0,9 mm/s and 5 =0,32 mm/s) which corresponds to MgFe204 and is responsible for the rests of the initial catalyst. It takes in a spectrum approximately 7% from the common area. This essential reduction of a doublet area in comparison with a spectrum of the initial catalyst allows to conclude that during the synthesis MgFe204 undergoes complex structural and chemical transformations leading to Fe3C, and carbon nanotube and nanofibres formation. Also in examined spectrum there are two doublet subspectra which spectral parameters analysis allows to attribute them to two Fe2+ nonequivalent positions in formed Mgi xFxO solid solution. The intensities ratio of these two doublets in Mossbauer spectrum allows to find the amount of the iron content in solid solution as x=0.15 [4]. Central singlet (with 5 = 0,07 mm/s) corresponds to y-Fe (C) with concentration of carbon in a sample about 1,5 % [5]. 

The data for mixed metal zeolites as first prepared by Scherzer and Fort (18) shown in Tables I and XI are quite extensive. The reported isomer shifts and quadrupole splittings are for the iron atoms in the anionic state. Each of these unreduced samples show Mossbauer spectra that are in close agreement with literature values of the corresponding iron coordination complexes. Typical examples of unreduced and reduced samples are shown in Figures 3 and 4. We note here that preparations 16 through 22 are new and are developments of our laboratory and that 9 through 15 are preparations based on the work of Scherzer and Fort (18). Samples 16 and 17 show that this method can be extended to other zeolites like ZSM-5. If no transition metal cation is used in the synthesis, no Mossbauer spectrum for the corresponding anion is observed. Therefore, the nature of the cation is critical and complexation of the anion to a cation is necessary for anion inclusion. Certain transition metal cations (Ru + for instance) do not seem to bind the anion. 

---