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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

F.A. Cotton in the early 1960s to refer to compounds containing metal–metal bonds. In another definition a cluster compound contains a group of two or more metal atoms where direct and substantial metal metal bonding is present [2]
.

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,

Fischer-Tropsch process
, although again iron-oxide based heterogeneous catalysts are used industrially.

Although discrete clusters have no well-defined role in industrial catalysis, they are widespread in Nature. Most prevalent are the

iron-sulfur proteins, which are involved with electron-transfer but also catalyse certain transformations. Nitrogen is reduced to ammonia at an Fe-Mo-S cluster at the heart of the enzyme nitrogenase. CO is oxidized to CO2 by the Fe-Ni-S cluster carbon monoxide dehydrogenase. Hydrogenases rely on Fe2 and NiFe clusters.[4]


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

oxidation states
.

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

di-tungsten tetra(hpp), the current (2007) record holder low ionization energy
.

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

photoelectron spectroscopy.[8][9] With an internal diameter of 6.1 Angstrom it is of comparable size to fullerene and should be capable of containing small atoms as in endohedral fullerenes
.

Gas-phase clusters and fullerenes

Unstable clusters can also be observed in the gas-phase by means of

Endohedral fullerenes
.

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

  1. ^ Inorganic Chemistry Huheey, JE , 3rd ed. Harper and Row, New York
  2. ^ Bioorganometallics: Biomolecules, Labeling, Medicine; Jaouen, G., Ed. Wiley-VCH: Weinheim, 2006.3-527-30990-X.
  3. .
  4. doi:10.1002/anie.200503916.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  5. ^ itself made by heating elemental potassium and lead at 350°C
  6. mass spectrometer
    before analysis
  7. doi:10.1021/ja062052f. {{cite journal}}: |format= requires |url= (help)CS1 maint: multiple names: authors list (link
    )
  8. doi:10.1103/PhysRevLett.93.023401.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  9. doi:10.1039/b614597c.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )

External links

  • http://cluster-science.net - scientific community portal for clusters, fullerenes, nanotubes, nanostructures, and similar small systems

See also

Category:cluster chemistry