Mixed-valence iron pairs

This cluster formally contains three iron(III) and one iron(E). It is present in a class of proteins called high potential iron-sulfur proteins (HiPIP). It has also been prepared through oxidation of [(RS)4Fe4S4]2 model compounds [57]. Both in the model compound at low temperatures and in proteins there is electron delocalization on one mixed valence pair [58-62]. Therefore, the polymetallic center is constituted by two iron ions at the oxidation state +2.5 and two iron ions at the oxidation state +3. Hamiltonian (6.20), or a more complicated one [40, 41,43], can be used to describe the electronic structure. Indeed, a delocalization operator is sometimes needed in the Hamiltonian [40,41,43]. Consistently with magnetic Mossbauer data the S M subspin involving the mixed valence pair is 9/2, whereas the S n subspin involving the iron(IH) ions is 4. Mossbauer and EPR data do not exclude % and 3, respectively, for the two pairs [57] in any case, the... 

The [Fe3S4] complex occurs naturally in a completely oxidized (3Fe ) and mixed Fe /Fe° oxidation states. Fully reduced forms of the three-iron clusters in either synthetic or natural systems have not been isolated. In mixed-valence clusters, electron delocahzation and antiferromagnetic pairing... 

Mixed-valence forms of the dinuclear iron unit have been observed for hemerythrin and MMO. These are characterized by EPR signals, which are observable only near liquid helium temperatures, with 1.7-1.8 and arise from the S = i state of an antiferromagnetically coupled high-spin Fe(III)-high-spin Fe(II) pair (Fig. 14) (127). Similar signals are found... 

Mother compound Lc Fe Os. Partial substitution of La " with Sr leads to compounds Laf+ Sr +Fef+ Fe 03+5 with mixed valence pairs (Fe /Fe ). Tentatively, in the compound Lao.75Sro.25Feg] Feg 03+, 50% of the iron cations have an heterovalence similar to neighbor-forming (Fe /Fe ) pairs. The other 50% of Fe " cations do not have a neighboring Fe ". ... 

ENDOR patterns as (basic) doublets separated by about 1 MHz and centered at about 16.5 MHz (mixed-valence) and 9.25 MHz (ferric pair). Therefore, as for the [2Fe2S] clusters, there should be considerable overlap with proton resonances when the measurements are taken at the usual X-band frequencies. Fortunately, the results for E. halophila II are shown in Figure 17 for three g-factors have better resolution than the ones discussed above for the [2Fe2S] clusters. Although there is no distinction possible between the individual irons within one tine group, the mixed-valence pair and the ferric pair are nicely distinguishable from the protons when the Fe sample is used for comparison (data not shown). Comparison with the available data based mostly on MSssbauer spectroscopy from several HiPIP... 

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