CLUSTERS redox behavior

L = axial ligands, n = 0, 1, 2) have been extensively investigated regarding their specific electronic, chemical, and physical properties [4], Particularly, oxo-centered triruthenium cluster complexes with bridging acetates attracted the most attention owing to their synthetic accessibility, multiple redox behavior, intriguing mixed-valence chemistry, and versatile catalytic properties [5-7]. 

Further studies on this system will include IR-SEC experiments under an atmosphere of CO to verify its catalytic activity for CO reduction and to aid in formulating a mechanism for the reaction. Other multimetallic systems used as CO reduction catalysts such as mthenium-, iridium-, and cobalt-based complexes, or metal clusters used as models in the active sites of biological systems, many of which have complex redox behavior can also be investigated using the IR-SEC technique. 

Four Fe/fS -like clusters appear to be present in [FeMo] and these have been designated as P-clusters( 5). The P clusters are, however, by no means ordinary Fe S clusters. If Indeed they are Fe S clusters at all. P clusters are conspicuous in UV-VIS and especially, MCD and Mossbauer spectra(6,13). The observed spectra are clearly not conventional. The clusters have a very high spin In their oxidized forms, probably S - 7/2 from recent(28a) EPR studies, and have decidedly inequivalent Fe populations(28). This implies that the putative Fe S/j clusters are highly distorted, presumably by unsymmetrlcal coordination from the protein. Moreover, the P clusters do not appear to behave Identically to each other under many circumstances. There is open disagreement as to the redox behavior of this set(6,2, 22). Furthermore, an additional Mdssbauer signal called S may in fact be part of the P-cluster signal(28). 

The interest in the redox, catalytic, and electrocatalytic properties of unsubstituted and substituted polyoxometalates arouses much attention [2-15] because they are a versatile family of molecular metal-oxide clusters with applications in catalysis as well as in medicine and material science. Such versatility must be traced to at least two main characteristics. First, the size and mass of these unique molecular oxides place their solution chemistry in an intermediate position between small molecule solution chemistry and infinite lattice solid-state chemistry. Second, their redox behaviors may be very flexible and finely tuned on purpose, by changing smoothly their composition, with a... 

The already voluminous review literature on clusters will be considered as a basis for this review. The topics treated so far are clusters in general (109, 241) and in connection with metal-metal bonding (30, 338, 380), special types of clusters like those with TT-acceptor ligands (231), hydrides (233), carbonyls (85, 86) or methinyl tricobalt enneacarbonyls (313, 317) properties of clusters like structures (56, 316), fluxionality (110), mass spectra (226), vibrational spectra (365), and redox behavior (292). Clusters have been treated in the context of metal carbonyls (3, 4), metal sulfur complexes (2, 381), and in relation to coordination polyhedra (297). Reviews... 

A number of excellent reviews have been written that address various aspects of metal-metal bonding. In particular, Cotton has contributed several eminent early reviews commensurate with his pioneering efforts in this area (54, 59, 60). More recently he reviewed quadruple bonds (56), molybdenum-molybdenum bonds (57), and multiple bond studies in his laboratory (55) in separate articles. Earlier reports discuss multiple metal-metal bonds in the context of general metal—metal bonding schemes and clusters at a time when the area was more limited in scope and therefore amenable to an encompassing survey (13, 128, 129, 164, 188, 189, 252). The redox behavior of metal-metal bonds has been reviewed by Meyer (193), and Walton has recently described ligand-induced redox reactions of strong multiple metal—metal bonds (253). 

Zanello, P. Stereochemical aspects associated with the redox behavior of heterometal carbonyl clusters, Struc. Bonding 1992, 79, 101. 

The electrochemistry of cluster compounds is still an underdeveloped area (75,76). A recent study has shown interesting differences in the redox behaviors of the isostructural compounds Fe4(CO),2Au2(PPh3)2BH and Fe4(CO),2Au2(AsPh3)2BH. Each undergoes a one-electron reduction and oxidation, but only the phosphine derivative can be oxidized further to the dication. All electrochemically generated species are short lived (71). 

The electrochemistry of HNCC is a new and rapidly expanding field which has not been covered in previous reviews of electrochemical studies of transition metal clusters 420). In a similar way to smaller clusters, HNCC have been found to exhibit both reversible and irreversible redox behavior. These processes will be discussed separately in the following sections. 

Further detailed studies of the electrochemistry of monocapped triangular clusters have been presented 223-225). The redox behavior depends on the capping group and the number of disparate metal atoms in the... 

The redox behavior of the F and H centers in hydrogenases I and II is nicely consistent with their respective modes of function. As shown in Figure 7.22, the F clusters are presumed to transfer electrons intermolecularly with the exter-... 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

In addition to the influence on E° values through the coordination environment, E° may be modulated by groups outside of the first coordination sphere. One example of this has been proposed to explain protein control of the redox behavior in Fe4S4 clusters. Three oxidation states are available to this cluster ... 

Guldi D M, Hungerbuhler H and Asmus K-D 1995 Unusual redox behavior of a water soluble malonic acid derivative of Cgg evidence for possible cluster formation J. Phys. Chem. 99 13 487-93... 

It often happens that predictions of the capacity of a cluster to exchange electrons are made simply on the basis of electron-counting rules, sometimes in conjunction with structural features. The aim of this review was to point out how the redox behavior of homo- and heterometallic clusters, even those which are isoelectronic and isostructural, can often be unexpectedly different. 

P. Zanello, Stereochemical Aspects Associated with the Redox Behavior of Heterometal Carbonyl, Clusters. Struct. Bonding (Berlin), 1992, 72, 101-214. 

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