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In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group elements form especially robust clusters.[1]
The phrase cluster was coined by
The main cluster types are "naked" clusters (without stabilizing ligands) and those with ligands. Typical ligands that stabilize clusters include carbon monoxide, halides, isocyanides, alkenes, and hydrides.
Applications of clusters in catalysis
Synthetic metal carbonyl cluster compounds have been evaluated as catalysts for a wide range of industrial reactions, especially related to carbon monoxide utilization,
Although discrete clusters have no well-defined role in industrial catalysis, they are widespread in Nature. Most prevalent are the
The term cluster should be pertinent to assembly of more than two metal atoms bound together in a planar or polyhedron arrangements such as Re3Cl9 and Mo6Cl8 units. Metal-Metal cluster could be classified as cages compounds or not when it is planar.
Electronic structure
Metal clusters are frequently composed of
The polyhedral skeletal electron pair theory or Wade's electron counting rules predict trends in the stability and structures of many metal clusters.
History and classification
The development of cluster chemistry occurred contemporaneously along several independent lines, which are roughly classified in the following sections. The first synthetic metal cluster was probably calomel, which was known in India already in the 12th century. The existence of a mercury to mercury bond in this compound was established in beginning of the 20th century.
Transition metal carbonyl clusters
The development of metal carbonyl compounds such as Ni(CO)4 and Fe(CO)5 led quickly to the isolation of Fe2(CO)9 and Fe3(CO)12. Rundle and Dahl discovered that Mn2(CO)10 featured an “unsupported” Mn-Mn bond, thereby verifying the ability of metals to bond to one another in molecules. In the 1970's, Paolo Chini demonstrated that very large clusters could be prepared from the platinum metals, one example being [Rh13(CO)24H3]2-.
Transition metal halide clusters
Boron hydrides
Contemporaneously with the development of metal cluster compounds, numerous boron hydrides were discovered by Alfred Stock and his successors who popularized the use of vacuum-lines for the manipulation of these often volatile, air-sensitive materials. Clusters of boron are boranes such as pentaborane and decaborane. Composite clusters containing CH and BH vertices are carboranes.
Fe-S clusters in biology
In the 1970s, ferredoxin was demonstrated to contain Fe4S4 clusters and later nitrogenase was shown to contain a distinctive MoFe7S9 active site.[5] With the development of bioinorganic chemistry, a variety of synthetic analogues of these clusters have been described.
Zintl clusters
Gas-phase clusters and fullerenes
Unstable clusters can also be observed in the gas-phase by means of
Extended metal atom chains
Extended metal atom chain complexes (EMAC) are a novel topic in academic research. They are comprised of linear chains of metal atoms stabilized with ligands. EMACS are known based on nickel (with 9 atoms), chromium and cobalt (7 atoms) and ruthenium (5 atoms). In theory it should be possible to obtain infinite one-dimensional molecules and research is oriented towards this goal. In one study [11] an EMAC was obtained that consisted of 9 chromium atoms in a linear array with 4 ligands (based on an oligo pyridine) wrapped around it. In it the chromium chain contains 4 quadruple bonds.
References
- ^ Inorganic Chemistry Huheey, JE , 3rd ed. Harper and Row, New York
- ISBN 0-13-479049-9
- ISBN 0-387-56470-5
- ^ Bioorganometallics: Biomolecules, Labeling, Medicine; Jaouen, G., Ed. Wiley-VCH: Weinheim, 2006.3-527-30990-X.
- ISBN 3-527-29549-6.
- doi:10.1002/anie.200503916.)
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External links
- http://cluster-science.net - scientific community portal for clusters, fullerenes, nanotubes, nanostructures, and similar small systems
See also
- Cluster (physics)
- Water molecules form clusters as well: see water clusters