Iron mixed valence compounds

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. 

There are many important mixed valence and fractional formal oxidation state compounds (e.g., Fc304) and complexes of iron. Mixed valence trinuclear /.t-oxo-carboxylato-complexes are dealt with in Section 5.4.5.4.2, polynuclear complexes held together by sulfide bridges in Section 5.4.5.9.2. 

Other Papers.—Various iron species prepared by the vacuum pyrrolysis of acetyl-ferrocene-furfural resins at 400°C have been studied by Mossbauer spectroscopy. These consist of an amorphous glass-like carbon matrix containing free iron atoms, Fe+ ions, iron clusters, superparamagnetic iron, and ferromagnetic iron.333 The effect of pressure of up to 50kbar on the absorption spectra of five iron(m), two iron(n) and one mixed valence compound has been studied. In six of the compounds, but not in basic ferric acetate or soluble Prussian Blue, the observed pressure-induced bands were assigned to d-d transitions of converted iron(n) for the ferric compounds and to spin-forbidden d-d bands for the ferrous compounds. The charge-transfer band from iron(n) to iron(m) in soluble Prussian Blue showed a blue shift at pressures up to 7.2 kbar.334... 

Ferric ferrocyanide, commonly known as Prussian blue (PB), was first synthesized > 300 years ago (39) and is still used in the manufacture of blueprints. Prussian blue is a prototypical mixed-valence compound with formula Fe 4[Fe (CN)6]3 I4H2O. In its canonical form, the pigment consists of ferrocyanide anions linked by Fe cations (Fig. 1) to form an extended pcu network. A defect structure that arises from the necessity of charge balancing in the cubic framework results in vacancies at 25% of the [Fe(CN)6]" sites (40). Analogous compounds are formed when one or both iron atoms are replaced by a variety of other metals. This substitution affords compounds of the formula M [My(CN)6]j where M and M can be Cr, Mn, Fe, Co, and many others, and where x andy depend on the identity and oxidation states of the metals. Because of their structural similarities we will refer to the entire class of compounds as blues PBs. 

Polynuclear transition metal hexacyanometallates, e.g., PB (iron(III) hexacyanoferrate(II)), are a class of insoluble mixed-valence compounds that have the general formula M j.[M"(CN)g] where M and M" are... 

The discussion in the preceding sections has been concerned with compounds which show a single valence or oxidation state. In this section some compounds which have iron in more than one oxidation state, usually iron(II) and iron(III), will be considered. Mossbauer spectroscopy can be very useful in determining the structural and electronic properties of such mixed valence compounds. 

Robin Day (1967) have classified mixed valence compounds on the basis of the eneigy required to transfer an electron between the low valence site and the high valence site. In Class I compounds, the ground state M(II)/ M (III) electronic configuration (where M and M refer to different iron(II) and iron(lll) sites in a compound) is much lower in eneigy than the excited state M(III)/M (II) configuration which may result from an intervalence electron transfer. In this type of compound the two iron sites may be... 

Not all mixed valence compounds show intervalence bands. Those that show not the slightest evidence for them are often called class 1. In class 2 intervalence compounds the intervalence-transfer band may well dominate the visible spectrum, swamping the spectra of the individual ions. However, these may be seen the important point is that the different ions retain their chemical individuality. Soluble Prussian blue is a case in point. This contains Fe and Fe bridged by CN ligands. Isotopic tracer measurements show that there is no doubt—the Fe" is bonded to the N atom of the CN ligand and the Fe to the C. If radioactive Fe is used in the preparation and the compound subsequently decomposed, the activity remains in the iron of the same valence state in which it was incorporated. A class 3 also exists, exemplified by the IJ anion, in which it is not possible to associate a unique valence state to individual metal ions typically, but as our example shows, not always, they are structurally indistinguishable. When they form an... 

Other well-known compounds that exist with a mixed valence are Prussian blue [Fe3+[Fen (CN)64-]], and ferre-doxin, in which iron has the valence of +11 and -Fill. 

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