Synthesis of Metal Clusters

The electron-rich metal clusters are most commonly prepared from metal compounds in which the transition metal has the most stable and common oxidation state. 

Reaction conditions are selected in order to synthesize either the cluster or a low-valent metal complex from which subsequently the cluster may be obtained by removal of some ligands. This type of synthetic methodology may be utilized for the preparation of most transition metal clusters. 

Polynuclear anionic metal carbonyl compounds are usually prepared by reduction reactions of metal carbonyls M(CO) with such reducing agents as the alkali metals, NaBH4 in ethers, hydrocarbons, liquid ammonia, and similar solvents [see, for example, reactions (2.54), (2.55), (2.84), (2.89), (2.95 -(2.102), and (2.108)-(2.113)]. In alkali medium the metal carbonyls may be reduced by certain solvents (e.g., alcohols) or by the CO ligand itself, and in the presence of Lewis bases the carbonyls disproportionate to give anionic clusters. The mixed metal clusters containing platinum and rhodium are formed by reduction reactions of chloro complexes 

Formally, the preparation of [PtRh5(CO)i5] may be described by the following stoichiometric equation  

The same cluster, [PtRh5(CO)i5], may also be obtained from anionic metal carbonyls of platinum and rhodium  

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Gates, B. C., Surface-mediated synthesis of metal clusters. J. Mol. Catal. 86,95 (1994). 

S. Fedrigo, T. L. Haslett, M. Moskovits, Direct Synthesis of Metal Cluster Complexes by Deposition of Mass-Selected Clusters with Ligand Iron with CO. J. Am. Chem. Soc. 1996, 118, 5083-5085. 

A. Ship-in-Bottle Synthesis of Metal Clusters in Zeolites and Clay Minerals Structures and Reactivity of Metal Clusters in Zeolites... 

Figure 2. Some examples of micro- and mesoporous materials as the template micro reactors for ship-in-a-bottle synthesis of metal clusters in micropores and mesoporous channels.

Ship-in-a-bottle synthesis of metal clusters in micropores... 

Figure 3. Pictorial illustration of ship-in-a-bottle synthesis of metal clusters, Rh6(CO)i6 assembled in NaY cages by the suecessive carbonylation of Rh ions using CO + H2O or CO + H2 as building blocks, which are introduced by the ion-exchange methods and gas admission.

G. Synthesis and reactivity of metal clusters G1. Synthesis of metal cluster complexes G2. Dynamic processes in metal clusters... 

The affinity of crown thioethers for heavy metals - and their antipathy to the biologically important ions, such as Na, Ca, and Mg -suits crown thioethers particularly well for decorporation of toxic metal ions. Similarly, crown thioethers may find potential use in the hydrometallurgical winning of precious metals such as silver, gold, and platinum, or as the basis for ion-selective electrodes [219]. Other potential applications [220] may arise from use of crown thioethers as structural building blocks or capping members (e.g. (M(9S3) ) for synthesis of metal cluster compounds. 

Examples given below illustrate synthesis of alkene polymerization catalysts, but these catalysts are simpler than the supported metallocenes used in industry, because they lack the promoter methylaluminoxane (MAO), an ill-defined material that greatly complicates characterization. Other examples given below illustrate (a) details of the surface chemistry of conversion of an organometallic precursor into a supported catalyst (b) synthesis of metal clusters of various sizes and compositions on a family of supports from metal carbonyl precursors and (c) synthesis of supported bimetallic clusters with combinations of noble (e.g., Pt) and oxophilic (e.g., W) metals that give quite stable catalysts with extremely high metal dispersions. 

For inducing the formation of metal-metal bonds, it is, in general, necessary for the metal atoms to exist in a rather low oxidation state. Especially suitable for the synthesis of metal clusters are therefore electron-deficient fragments in which the metal atoms have the same oxidation state than that they will have in the cluster. These fragments are normally very unstable and labile species. Electron deficiency induces a coordinative unsaturation that in absence of strong donor species will tend to form metal-metal bonds. 

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