Nuclearity of clusters

The use of terminal CN functionalities represents an attractive means to extend the nuclearity of clusters. The pentanuclear TBP complexes described in Section II. D contain six dangling cyanide ligands (Fig. 37) that can be used for the attachment of additional metal ions. Mallah and co-workers (216) successfully employed this strategy by starting with the mononuclear precursors, [(i-Prtacn)Ni Cl2] and K3[Cr CN)6]. The product was precipitated by the addition of NaBPh4, and... 

It has, for example, been found that during the very slow coalescence of Ag inside the cavities of a Nafion membrane, the smallest clusters could be oxidized by the protons H3O+, which are highly concentrated at the surface. In contrast, when, by coalescence, the clusters reach the critical nuclearity for which their potential is higher than °(H30 /H2) = 0 Vnhe, they escape corrosion and are observed by optical absorption. Numerical simulation of the kinetic signal, including the cascade of coalescence reactions (Eq. 11) and of oxidation reactions (Eqs. 35 37), yields the value n = 8 for the upper limit of nuclearity of clusters oxidized by HjO+.t " Therefore, °(Ag,o+/Agio) > °(H30+/H2) = 0 Vnhe (Fig. 11). Note that such a corrosion by H3O+ was not observed under conditions of free diffusion of the clusters, as in solution,because the coalescence enables the clusters to grow much faster up to the supercritical nuclearities. 

IR bands from CO adsorbed on Pt(l 11) showed the formation of terminal CO on terrace sites (near 2091 cm ) at 90 K. At 360 K, there was evidence for terminal CO on step-sites (near 2074 cm ). IR spectroscopy was also used to follow the behaviour of CO on Pt l 10 -(lx2) surfaces in the temperature range 30-70 Several other studies have been made on adsorption of CO on Pt surfaces. Values of vCO were used to follow the changing nuclearity of clusters [Pt3(CO)6 ]n, where n = 2, 3, 5 or - lO, under the influence of acids and bases on an AI2O3 surface. 

Despite the ability of ESR spectroscopy to identify unambiguously the presence and nuclearity of clusters, the technique is not ideally suited to providing... 

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

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. 

Reactions of clusters with mononuclear or dinuclear metal complexes frequently provide a method of expanding the metal core nuclearity under controlled conditions. The majority of medium- and high-nuclearity homometallic clusters has been prepared from lower-nuclearity cluster precursors by thermolyses ("heat-it-and-hope ) reactions. This is less true of the heterometallic clusters in this... 

Park S, Wasileski SA, Weaver MJ. 2001. Electrochemical infrared characterization of carbon-supported platinum nanoparticles A benchmark structural comparison with single-crystal electrodes and high-nuclearity carbonyl clusters. J Phys Chem B 105 9719 -9725. 

The nuclearity of structurally characterized homonuclear clusters goes from four to 39. The preparation of [Au55Cl6(PPh3)i2] from the reduction of [AuCl(PPh3)] with B2H6 has also been reported.46,3217... 

A series of tri- and tetra-nuclear Ru clusters previously reported by the groups of Siiss-Fink [60] and of Dyson [61] as catalysts for the hydrogenation of benzene and other simple aromatics in biphasic media have later been shown to consist predominantly of active metallic particles [9, 10, 62]. 

Metal bromides, 4 322-330 Metal can food packaging, 18 37-39 Metal-carbene complexes, 26 926 Metal-carbon compounds, 4 648, 650 Metal-carbon eutectic fixed points, 24 454 Metal carbonyl catalysts, supported, 16 75 Metal carbonyl complexes, 16 73 Metal carbonyls, 15 570 16 58-78 bonding and structure of, 16 59-64 from carbon monoxide, 5 12 in catalysis, 16 72-75 economic aspects of, 16 71 health and safety aspects of, 16 71 heteronuclear, 16 69-71 high nuclearity, 16 66-69 high nuclearity carbonyl clusters, 16 64-66... 

Ozin GA, Flugues F, Mattar SM, McIntosh DF (1983) Low nuclearity silver clusters in faujasite-type zeolites optical spectroscopy, photochemistry and relationship to the photodimerization of alkanes. J Phys Chem 87 3445-3450... 

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