Hydride metal clusters

NANOSIZED CLUSTERS or nanoclusters is a term given to particles of any kind of matter, the size of which is greater than that of typical molecules, but is too small to exhibit characteristic bulk properties. Examples of such materials include the family of carbon-based fullerenes, weakly bound van der Waals clusters observed in the gas phase, metal carbonyl clusters, boron carbides and hydrides, metal clusters surrounded by protecting ligands, and metal ions connected by bridging ligands. Of special... 

There are numerous triangular metal complexes containing hydrogen. Structural features of hydride metal clusters will be separately described in Chapter 3. The study of cluster hydrides is of considerable interest since, as it will be discussed later in this Chapter (Sect. 2.6), they are intimately associated with important catalytic processes e.g. Fischer Troppch processes for ammonia and methane synthesis. 

The oxidative addition of hydrogen to cluster is the best known reaction of this type. They are widely used for preparing hydride metal clusters. 

Covalent. Formed by most of the non-metals and transition metals. This class includes such diverse compounds as methane, CH4 and iron carbonyl hydride, H2Fe(CO)4. In many compounds the hydrogen atoms act as bridges. Where there are more than one hydride sites there is often hydrogen exchange between the sites. Hydrogens may be inside metal clusters. 

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. 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

The commonly investigated ligands on the supported metal clusters mentioned before include CO, hydride, and hydrocarbons. The evidence of hydrides and hydrocarbons is not as strong as one would wish. 

Al, Ga, In and T1 differ sharply from boron. They have greater chemical reactivity at lower temperatures, well-defined cationic chemistry in aqueous solutions they do not form numerous volatile hydrides and cluster compounds as boron. Aluminium readily oxidizes in air, but bulk samples of the metal form a coherent protective oxide film preventing appreciable reaction aluminium dissolves in dilute mineral acids, but it is passivated by concentrated HN03. It reacts with aqueous NaOH, while gallium, indium and thallium dissolve in most acids. 

This chapter presents results of NMR studies of several heavy metal hydrides, including both a summary of completed work by Lau et al. (i) on a binary hydride, Th4His, and preliminary results on several carbonyl hydrides, H2Os3(CO)io, H4Os4(CO)i2, and H4Ru4(CO)i2. The binary hydride, Th4Hi5, has attracted interest recently with the discovery of its super-conducting properties (2) and the carbonyl hydrides are metal cluster hydrides (3) which are of interest as models in the study of catalysis (4). 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Sometimes, reactions in which an alcohol is oxidized by hydride transfer to a metallic cluster, resulting in the formation of a metallic hydride that subsequently transfers a hydride to a sacrificial aldehyde or ketone, are described as Oppenauer oxidations.44 In the opinion of the authors, the name Oppenauer oxidation should be reserved for oxidation of alcohols in which a hydride is directly transferred from a metallic alkoxide to an aldehyde or ketone acting as oxidant. 

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