Metal clusters stability

As was shown in previous section, the many-body forces play a crucial role in metal cluster stability. So, a model potential must include many-body terms, at least 3- and, sometimes, 4-body ones. For clusters of larger size, the fitted parameters in these terms will include ( absorb ) many-body effects of higher orders. 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

Recently a technique for the preparation of catalyst particles with a narrow size distribution was developed [8], yielding colloidal metal clusters stabilized by a shell of surfactants. By adsorbing these clusters on substrate surfaces, model electrodes for dispersed electrocatalysts can be prepared [9]1 Figure 2 compares two samples prepared from different colloidal solutions of such clusters adsorbed on a gold surface. It is evident that both samples differ significantly wifii respect to their mesoscopic structure. 

Another set of fundamental properties of metal clusters involves their response to static external electric and magnetic fields. Transition metal clusters embedded in matrices have been extensively studied with these techniques. [143] Unfortunately, the size distribution of the particles is broad in these experiments and interactions with the matrix can introduce changes in the properties of the metal aggregates. This is also true for metal clusters stabilized by ligands, as will be discussed in Section 2.4.5.3. The study of clusters in molecular beams overcomes these difficulties. This powerful approach, when combined with mass spectrome-tric detection, allows the investigation of mass selected free clusters. The main disadvantage is that since the particle densities are quite low, most of the standard spectroscopic techniques cannot be used. 

The structures of the alkali metal suboxides have been discussed in terms of bare metal clusters stabilized by interstitial oxygen atoms. Due to the direct M-M bonding between the clusters, the suboxides are metallic and show electrical properties [220] which are similar to the free alkali metals. 

Under metal/carbon nanocomposite we understand the nanostmcture containing metal clusters stabilized in carbon nanofilm stmctures. The carbon phase can be in the form of film stmctures or fibers. The metal particles are associated with carbon phase. The metal nanoparticles in the composite basically have the shapes close to spherical or cylindrical ones. Due to the stabilization and association of metal nanoparticles with carbon phase, chemically active metal particles are stable in air and during heating as the strong complex of metal nanoparticles with the matrix of carbon material is formed. The test results of nanocomposites obtained are given in Table 1.1. 

Khama, S. Dasgupta, S. Biradha, K. Bhattachaijee, M. Self-assembly of an alkali metal cluster stabilized by a new flexidentate metalloligand formation and structure of heterobimetalhc Na-Mo and Cs-Mo 2D networks. Eur. J. Inorg. Chem. 2005,5005. 

Supported metal clusters are important for catalytic applications and have therefore attracted attention for many years. Noble metals have been introduced into zeolites by ion exchange of complex cations such as Pd(NH3)4 +. Degassing in oxygen eliminates premature autoreduction, and subsequent hydrogen reduction forms very small metal clusters stabilized by the zeolite structure, (see example of Pd zeolites below). Other, more powerful reducing agents including H-atoms and sodium vapor have been used for metals such as Ni or Fe. Often a particle size distribution with additional external phase is obtained. 

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

In coating fullerenes with alkali metals, the stability of the cluster seemed to be determined primarily by the electronic configuration. The units C qM and C7oMg, where M is any alkali metal, proved to be exceptionally stable cluster building blocks. Coating a fullerene with more than 7 alkali metal atoms led to an even-odd alternation in the mass spectra, inter-... 

Numerous examples oF metal carbonyls will be found in later chapters dealing with the chemistry of the individual transition metals. CO also has an unrivalled capacity for stabilizing metal clusters and for inserting into M-C bonds (p. 309). Synthetic routes include ... 

A method has recently been described for wrapping polymers around metal atoms and very small metal clusters using both matrix and macroscale metal vapor-fluid polymer synthetic techniques. Significant early observations are that (i) the experiments can be entirely conducted at, or close to room temperature, (ii) the resulting "pol5aner stabilized metal cluster combinations are homogeneous liquids which are stable at or near room temperature, and (,iii) the methodology is easily extended to bimetallic and trimetallic polymer combinations. ... 

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

The octahedron is classified into the c/o o-structure by Wade [3,4]. Closo-structures with n skeletal atoms are stable when they have 4n-i- 2 valence electrons. Wade s rules predict that the 26 (= 4 x 6 + 2) valence electrons could stabilize the regular octahedrons since n is 6 for the octahedron. This prediction is contained in our 6N + 14 (N= 2) valence electron rule. Our rule also predicts the stability of octahedral metal clusters with the other numbers (14 and 20) of valence electrons. 

The definitions of the cluster ligands of the [3Fe-4S] cluster and of the related structural features are quite useful to predict cluster types in other Fds of known sequence, as well as to determine the nature of the cluster coordinating atoms (and variability) and their control on the type and performances of the metal sites, in particular in terms of cluster stability, cluster interconversion capability, and acceptance of other metals at the cluster. 

Exploratory solid state synthesis seems to be the only workable route to new phases because of a general inability to predict relative phase stabilities and thence structures or compositions , published in K4La6li40s A new Structure Type for Rare-Earth-Metal Cluster Compounds that Contain Discrete Tetrahedral K4l Units. S. Uma, J.D. Corbett, Inorg. Chem. 1999, 38, 3831-3835. 

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