Metal cluster compounds bimetallic clusters

A variety of metal cluster compounds have been chemically bound on amorphous metal oxides and entrapped inside zeolite cages by new preparative tools such as surface organometallic chemistry and the so-called ship-in-bottle technique. They oflier much promise as molecular precursors for rational preparation of tailored metal catalysts having a uniform distribution of discrete metal-bimetallic ensembles, namely, organometallics which are active for catalytic reactions. They also provide advantages as metal precursors to achieve higher metal dispersions and well-managed metal... 

The subject of heteronuclear cluster compounds of the transition metals remains an active area of research interest, and was reviewed in the early 1980s by Geoffroy el al. (1,2). Clusters with novel architectures, exemplified by the star clusters of Stone and co-workers (5), continue to be synthesized. Whereas there is undoubtedly strong academic interest in the structure, bonding, and chemical reactivity of heteronuclear clusters in their own right, additional impetus to this field is given by the important relationship between heteronuclear clusters and bimetallic alloy catalysts. This relationship was the subject of a published symposium (4). 

A few illustrative examples dealing with bimetallic cluster compounds will now be presented. For example, mixed cluster compounds [MjRu -CO CO qCPPI J where M = Cu, Ag, and Au have been prepared and structurally characterized. ) Metal core structures consist of a Ru4 tetrahedron with M-PPh3 groups capping two... 

Supported bimetallic catalysts can be made by adsorption of a bimetallic precursor such as molecular cluster compounds, colloidal particles or dendrimer-stabilised particles. In several cases, homogeneous bimetallic particles have been found where the compositions lie within the miscibility gap of the bulk alloy (e.g. with PtAu particles). This suggests that when the particles are small enough and do not possess metallic properties, the normal rules do not apply. 

Multimetallic sandwiches, including polymetallocenes, bis(fulvalene)-dimetal compounds, multi-decker sandwiches, and compounds in which two metals are bonded to a common unsaturated cyclic hydrocarbon are discussed in Section II. Sections III and IV are subdivided according to the metal group, and describe ligand-bridged bimetallics and metal clusters, respectively. Where appropriate, redox potentials have been included. Unless otherwise stated they are referenced versus the aqueous saturated calomel electrode (see). 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

Metal clusters and colloids cannot be isolated in an unprotected form, as coalescence processes set in immediately by contact between particles to give amorphous or polycrystalline powders. Therefore, colloids have always been used in highly diluted dispersions, in polymers or in matrices [1]. Clusters are well known as stable compounds if they are protected by a shell of appropriate ligands. To make colloids available as isolable molecules (e. g. for homogenous catalysts), one developed a method to stabilize them by a ligand shell similar to that of clusters [2-4]. Colloids thus became applicable for numerous chemical and physical investigations [2, 5]. Even bimetallic particles have been stabilized and made useful in various practical applications [5]. 

Bimetallic cluster compounds containing coinage metals and peripheral Fe(CO)4 groups have been synthesized and show a considerable structural variety. Particularly interesting are the clusters of composition [M4 Fe(CO)4 4]" (M = Ag, Au). Clusters of other stoichiometry have been isolated with M = The metal... 

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