In chemistry, a cluster is an array of bound atoms intermediate in character between a molecule and a solid. Carbon and boron atoms form fullerene and borane clusters. The other main group of clusters are metal clusters Inorganic Chemistry Huyee, JE , 3rd ed. Harper and Row, New York.

The phrase cluster was coined by F.A. Cotton in the early 1960's as 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 Introduction to cluster chemistry by D. M. P. Mingos, David J Wales 1990 ISBN 0134790499.

The main cluster types are naked clusters without stabilizing ligands or those with ligands such as carbon monoxide, halogens, isocyanates and ethylenes.

Cluster compounds find use as catalysts for a wide range of reactions Cluster Chemistry: Introduction to the Chemistry of Transition Metal and Main Group Element Molecular Clusters Guillermo Gonzalez-Moraga 1993 ISBN 0387564705. For example Ru₃(CO)₁₂ and Ir₄(CO)₁₂ are catalysts for the Water gas shift reaction and Rh₆(CO)₁₆ one for the Fischer-Tropsch process.

The polyhedral skeletal electron pair theory or Wade's electron counting rules predict closed-shell requirements for clusters.

Metal clusters

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The oldest identified metal cluster is 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.

Metal cluster are prominently found with refractory metals. In general metal centers with large overlapping d-orbitals form stable clusters. Thus metals with a high oxidation state and therefore large effective charge tend to be unstable. Polynuclear metal carbonyls are generally found in late transition metals with low formal oxidation states. Polynuclear halides and oxides are found with early transition metals.

The development of metal carbonyl compounds led quickly to the isolation of Fe₂(CO)₉ and Fe₃(CO)₁₂. Linus Pauling characterized “MoCl₂” to contain Mo₆ octahedra. 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.

The CO ligand can add to the polynuclear complex isthree different ways:

• two-center or terminal carbonyl (μ₁ CO) in which the CO molecule is bonded to one vertex only. The M-CO bond can be linear or bent.
• three-center or edge-bridging carbonyl (μ₂ CO) in which CO bonds to two metal centers.
• four-center or triangular face-bridging (μ₃ CO) in which CO bonds to three metal centers sharing a face.

An often studied metal cluster compounds is dinuclear potassium octachlorodirhenate(III) or K₂Re₂Cl₈ whose peculiar molecular structure is explained by Quadruple bonding. Another dinuclear compound is di-tungsten tetra(hpp), the currect record holder low ionization energy.

In trinuclear rhenium(III) chloride or (ReCl₃)_{3 the metal centers bond directly and through chlorine bridges. Albert Cotton established that ReCl3 was in fact the cluster Re3Cl9, which could be converted to a host of adducts without breaking the Re-Re bonds.}

Because this compound is diamagnetic and not paramagnetic the rhenium bonds are double bonds and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in hydrochloric acid a Re₃Cl₁₂^3- complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W₄(OCH₃)₁₂ with tungsten single bonds and molybdenum chloride (Mo₆Cl₈)Cl₄ is a hexanuclear molybdenum compound and an example of an octahedral cluster. A special group of clusters with the general structure M_xMo₆X₈ such as PbMo₆S₈ form a Chevrel phase which exhibit superconductivity at low temperatures.

Naked clusters

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naked clusters also called Zintl compounds form a separate class of metal clusters. they form when poor metals are dissolved in liquid ammonia in the presence of alkali metals. Examples of Zintl anions are Bi₃^3-,Sn₉^4-,Pb₇^4- and Sb₇^3-. These anions do not require ligands and called naked clusters but are unstable nevertheless when not stabilized as salts by cryptates. The structure of the Pb₁₀^2- anion is that of a capped square antiprism The Zintl Ion A Rare Example of a Homoatomic closo Cluster Annette Spiekermann, Stephan D. Hoffmann, Thomas F. Fässler Angewandte Chemie International Edition Volume 45, Issue 21 , Pages 3459 - 3462 2006 [http://dx.doi.org/10.1002/anie.200503916 Abstract. According to Wade's rules (2n+2) the number of cluster electrons is 22 and therefore a closo cluster. The compound is prepared from oxidation of K4Pb₉ (itself made by heating elemental potassium and lead at 350°C) by Au⁺ in PPh₃AuCl (by reaction of Hydrogen tetrachloroaurate and triphenylphosphine) in ethylene diamine with 2.2.2-crypt. This type of cluster was already known as is the endohedral Ni@Pb₁₀^2- (the cage contains one nickel atom).

Bioinorganic clusters

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In bioinorganic chemistry in the 1970's, ferredoxin was demonstrated to contain Fe₄S₄ clusters and later nitrogenase was shown to contain a strikingly distinctive MoFe₇S₉ active site.

Transcient clusters

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Many metal cluster molecules can only be observed by means of mass spectroscopy or X-ray photoelectron spectroscopy but not isolated. Examples of such clusters are certain aluminium clusters as superatoms and certain gold clusters. In certain metal clusters metal aromaticity is observed.

Carbon and Boron clusters

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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 volatile, air-sensitive materials. Clusters of boron are boranes such as pentaborane and decaborane.

Clusters of carbon are fullerenes and nanotubes. The fullerene sphere can be filled with small molecules in Endohedral fullerenes.

Composite clusters of carbon and boron are carboranes.

References

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cluster chemistry