Cluster nuclearity

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

Ir4(CO)i2 reacts with the surface of MgO to generate surface species in which the tetrahedral metal framework is preserved. The structures obtained after decar-bonylation under H2 at 573 K depend on the degree of hydroxylation of the support The iridium cluster nuclearity of 4 was maintained for a low degree of MgO hydroxylation (MgO pretreated at 973 K), but it increased to 6 when the MgO was highly hydroxylated (MgO pretreated at 573 K) [206, 207]. The activity in propane hydrogenolysis of the tailored catalyst is two orders of magnitude less than that of the conventional catalyst at atmospheric pressure and 200 °C. 

High-nuclearity carbonyl clusters, 30 123-222, see also specific metal and clusters decrease in cluster nuclearity, 30 152 interstitialhydrido, NMR data, 30 168 large hydrido clusters, structure, 30 168-172... 

Since mechanistic information for cluster expansions is scarce it cannot be excluded that all such reactions described so far proceed via the addition of coordinatively unsaturated mononuclear complex fragments, even though they can be formulated differently. It is likely that most uncontrolled reactions changing the cluster nuclearity do proceed this way. However, there are a small number of simple cluster expansions which can be understood best by assuming intermediate fragments. This was already taken into account in the previous paragraph. It holds for the following reactions between clusters and simple complexes which do not bear an obvious center of reactivity. 

The combination of a cluster expansion reaction with removal of a mononuclear fragment results in a net metal exchange reaction retaining the overall cluster nuclearity. This simple process inevitably has a very complex mechanism if performed as a one-step synthesis. Nevertheless such reactions have been found for trinuclear clusters and systematized for three different kinds of reagents (16,178). 

Figure 2.1 Representations of the structures of some Mo-based POM clusters (nuclearity given in subscript), all synthesized under...

Changes in cluster nuclearity have also been observed to accompany the addition of acid or base to heteronuclear gold cluster compounds and may result in aggregation, as in the reactions... 

It has become evident that as cluster nuclearity increases, the simple relationship between their electron count and structure is lost, particularly... 

As the cluster nuclearity increases, the structures and electron counting of saturated clusters conform to the polyhedral skeletal electron pair (SEP) theory. For organometallic clusters the carbon fragments of the hydrocarbyl ligands are frequently part of the cluster skeleton. Thus, Fc3(PhCCPh)(CO)9 (1) would be considered to be unsaturated according to the 18-electron rale, as the alkyne ligand is a four-electron donor to a Fc3(CO)9 cluster, which requires... 

The interaction of TM metal clusters with the surface of MgO has been studied with ab initio methods with both cluster [201-204] and slab [205-207] models but mostly small supported clusters, containing less than 10 atoms, have been considered. The theoretical study of these systems is rather complex because of the very many isomers that one can have. The number of these isomers increases exponentially with the cluster nuclearity making an accurate evaluation with ab initio methods of the relative stabilities very difficult and extremely time consuming. The metals studied so far are Co, Ni, Cu, Pd, Ag, and Au. Some of isomers studied for Co, Ni, Cu, Pd, Ag tetramers are shown in Fig. 2.10. They can be classified into three main groups Planar or nearly planar structures with the cluster plane parallel to the MgO surface, Fig. 2.10a-c, planar structures with the cluster plane normal to the surface, Fig. 2.10d-f and tetrahedral or distorted tetrahedral structures (Fig. 2.10g-i). 

Alumina supported ruthenium clusters were studied for the effect of cluster nuclearity on the rate of CO2 methanation. It was found that the reactivity paralleled the nuclearity of the catalyst precursor. 

Gofflot, F., Chartoire, N., Vasseur, L., Heikkinen, S., Dembele, D., Le, M. J., and Auwerx, J. (2007) Systematic gene expression mapping clusters nuclear receptors according to their function in the brain. Cell 131, 405 418. 

A charged cluster may constitute an electron acceptor, but that depends on its own redox potential value, E (A -Agn) relative to the threshold imposed by the monitor potential, E°(Q -QH2). As the redox potential increases with cluster nuclearity (5, 6), a certain time after the pulse is required to allow the first supercritical clusters to be formed and their potential to reach the threshold value imposed by the hydroquinone. When time, t, is less than tc, where n < Uc, the transfer is not allowed. During this induction period, the kinetics at 380 nm correspond to pure coalescence of clusters (Figure 4), and hydroquinone is stable (the bleaching OD512 is constant). That means, obviously, that none of the silver species present at that time can react with hydroquinone, especially free Ag ions and Ag ions associated with the smallest clusters. 

At lirst glance, the tendency noted by Dykstra 110 for RMg X—decreasing stability with increasing cluster nuclearity—seems to be reproduced for HMg H derivatives (good models for RMg H derivatives, which should exhibit the same trends). For both the hydride and halide derivatives, however, this decrease in stability with increasing nuclearity is an artifact of the calculation. The stability of linear structures indeed decreases with increasittg nuclearity. But linear structures appear to be most stable only for n < 3. The situation changes for n = 4, for which a rhombic... 

G. A. Ozin and C. G. Francis, J. Mol Struct., 59, 55 (1980). The same experiments can be earned out at 290 K using a meta-substituted pentaphenyl ether (Santovac-5) manufactured by Monsanto. The spectroscopic properties of the products up to a vanadium cluster nuclearity, n - 3 (microsolution) or n = 2 (macroscale), are identical to those of the DC510 supported vanadium. 

With the even bulkier Bu SlT, the reaction produced dinuclear species containing a diamond-shaped dinuclear core bridged by two /i-SBu ligands. The difference in cluster nuclearity can be rationalized in terms of the steric hindrance provided by the thiolate ligands. When a sterically... 

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