Trapped-valence

A representative structure is shown in Figure 3. In all cases, Mn...Mn separations are long (ca 3.3A), similar to values found for the mono-oxlde-brldged Mn2 complexes described above. Even in the trapped-valence complex Mn30(02CPh)s(py)2(H20) (18) shown in Figure 3, where the Mn3 unit is isosceles rather than equilateral, the Mn...Mn separations are not distinctly different, and no separation is found in the ca 2.7k range. It could thus be concluded that an MnsO-type unit cannot represent the complete... 

Mossbauer studies of the electron transport protein ferredoxin n from Desulfovibrio gigas have shown that the reduced Fe3S4 cluster contains one trapped valence Fe3+ site and one delocalized Fe2+-Fe3+ pair. The two Fe of the delocalized pair are indistinguishable, and the pair has a dimer spin S12 = 9/2, suggesting ferromagnetic coupling. 

Figure 3. MOssbauer spectra of the reduced Rieske protein Thermus Thermophilus. (A) Spectrum taken at 230 K. The brackets indicate the doublets of the trapped-valence Fe2+ and Fe3+ sites. (B) 4.2 K spectrum of the same sample. The solid line is a spectral simulation based on an S = 1/2 spin Hamiltonian. S = 1/2 is the system spin resulting from coupling S = Sa + Sb according to H = JSa-Sb for J > 0 Sa = 5/2 and Sb = 2.

An X-ray photoelectron spectroscopic study of Ni(DPG)2I showed no evidence of trapped valence or any appreciable change in the charge on the metal upon oxidation.97 The site of partial oxidation and hence the electron transport mechanism is still unclear but one explanation of the relatively low conductivity is that the conduction pathway is metal centred and that the M—M distances are too long for effective orbital overlap. Electron transport could be via a phonon-assisted hopping mechanism or, in the Epstein—Conwell description, involve weakly localized electronic states, a band gap (2A) and an activated carrier concentration.101... 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

The photoelectron spectrum was initially interpreted (29e) in terms of trapped valencies but a theoretical study by Hush (67) has shown that a delocalized description is equally consistent with the data available so far. Infrared data are still under discussion, but the Raman spectrum has been interpreted (29f) in terms of localization. It does seem clear that the rate of valence interchange according to Eq. (1) is rapid compared with the NMR time scale (29b) the possibility remains that the rate may be comparable with the frequencies of certain bond vibrations. 

This ion provides a model for study of intervalence transfer absorption bands characteristic of trapped valence species that correspond to the process... 

When ferrocenyl groups are linked together without insulating bridges, one oxidation wave per iron atom is observed. Thus, l,l -terferrocene 5 and l,l -quaterferrocene 6 show three and four oxidation waves, respectively (72). Increasing the chain length leads to a more facile first oxidation [ values of 0.40 V for ferrocene, 0.31 V for 1 (M = Fe), 0.22 V for 5, and 0.16 V for 6], and smaller separations between the first two values [A values of 0.34 V for 1 (M = Fe), 0.22 V for 5, and 0.20 V for 6], consistent with electronic delocalization between metal centers despite their formally trapped valences. 

Theoretical studies have shown that the monocations of both biferrocene and bis(fulvalene)diiron show some delocalization the latter is much more delocalized than the former (202). By contrast, [Feairj, -CsH4CH(Me)C5H4 2] appears to be a trapped valence cation X-ray structural studies have revealed different environments about the two metal atoms (203) (Section II,A). 

Farver O, Hwang HJ, Lu Y, Pecht I. (2007) Reorganization energy of the Cu center in purple azurin Impact of the mixed valence-to-trapped valence state transition. J Phys Chem Sill 6690-6694. 

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