Mixed valence clusters

In summary, the Mossbauer data presented by Fee et al. (5) gave the first conclusive evidence that Rieske clusters contain noncysteine ligands bound to the Fe" site of a localized mixed valence cluster. In addition, strong similarities with [2Fe-2S] clusters in bacterial dioxy-... 

The electrochemical and spectroscopic data indicates that sites on these polymers can communicate with each other, in the electron transfer sense, on a relatively short time scale and without the formation of stable mixed valence clusters. Electronic tranport via hopping or tunnelling and modulated by means of neighboring molecular group collisions would be consistent with these requirements. The relative molecular nonspecificity of this mechanism suggests that other polymeric materials would show similar effects and this has been seen for thin films of poly — (vinylferrocene) and poly — (nitrostyrene). 

Finally, mixed-valence cluster-containing polymers were also synthe-sized.40,42,43 Indeed Pd- or Pt-centers exhibiting a valence of +0.5 were used. Palladium-containing polymer 39 (Fig. 21) incorporates Pd4 arrays, which are linked one to another by a dmb ligand, whereas the two Pd atoms inside the Pd4 unit are coordinated by two dmb molecules each. The analogous Pt4 polymers 40—42 were also prepared. The main difference is that the linking dmb unit is replaced by a diphos ligand (Fig. 21). 

Other electron-acceptor ligands can take the place of CO, for example S02. There is a series of higher clusters Pd (CO)x(PR3), (n = 3-8, 10, 16, and 23) they form on reduction of Pd salts in the presence of CO and PR3. Mixed-valence clusters may also arise in a self-assembly fashion, for example, (18-H-III) is formed from palladium acetate, CO, H+, and Ph2PCH2PPh2 (dppm).4... 

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... 

Figure 5 Molecular structure of the mixed-valence cluster (40)...

The multiline spectrum shows some resemblance to that of a mixed valence cluster Mn - Mn or Mn - Mn [88,89,97] and with a computer simulation of a tetramer spectrum such as 3Mn - Mn [90]. Precise agreement, however, is lacking so far. The form and number of the lines depend on the period of dark adaptation and the illumination temperature and are sensitive to the presence of inhibitors of O2 evolution [91,92]. The signal is orientation-dependent, i.e. it has a fixed geometry with respect to the membrane [93]. 

Krockel M, Grodzicki M, Papaefthymiou V, Trautwein AX, Kostikas A (1996) Tuning of electron delocalization in polynuclear mixed-valence clusters by super-exchange and double exchange. J Biol Inoig Chem 1 173-176... 

High-Nuclearity Mixed-Valence Clusters Localization vs. Delocalization of the Electronic Pair in the Double Reduced Polyoxometallates with Keggin Structure... 

Charge and Structural Ordering in Crystals Comprising Mixed-Valence Clusters... 

Jones and co-workers reported the fully characterized neutral mixed valence cluster [Pd3(Cl)(/r- Bu2P)3(CO)2], Pd3(4/3). Reaction of [PdCl(CO)] with Li- Bu2P in THF at —78 °C produces a mixture of compounds (Eq. 6) from which the trinu-clear phosphido-bridged complex was isolated in low yield. 

The species can be subdivided into four main groups, three of which consist of stoichiometric compositions [Cu2 Te (TeR)2x(PR2R )m] that differ in the nature of their tellurium ligands, while the fourth group represents the mixed-valence clusters. 

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