Cluster chemistry

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. .

The phrase cluster was coined by F.A. Cotton in the early 1960s 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.

The main cluster types are naked clusters without stabilizing ligands or those with ligands. Typical ligands that stabilize clusters include carbon monoxide, halides, isocyanides, alkenes, and hydrides.

Applications of clusters in catalysis
Synthetic cluster compounds were once proposed to be useful as catalysts for a wide range of industrial reactions, especially related to carbon monoxide utilization, but few industrial exist. The clusters Ru3(CO)12 and Ir4(CO)12 are catalysts for the Water gas shift reaction, also catalyzed by iron oxide, and Rh6(CO)16 catalyzes the Fischer-Tropsch process, although again iron-based heterogeneous catalysts are used industrially.

The major role of clusters in catalysis are found in Nature. Nitrogen is converted to ammonia via the Fe-Mo-S cluster at the heart of the nitrogenase. CO is oxidized to CO2 by carbon monoxide dehydrogenase. Hydrogenases rely on Fe2 and NiFe clusters.

Electronic structure
Metal cluster are prominently found with refractory metals. In general metal centers with large d-orbitals form stable clusters because of favorable overlap of valence orbitals. Thus, metals with a low oxidation state and therefore small effective charges tend to form stable clusters. 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 polyhedral skeletal electron pair theory or Wade's electron counting rules predict trends in the stability and structures of clusters.

History
The oldest identified 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.

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. F. 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. 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. In the 1970s, ferredoxin was demonstrated to contain Fe4S4 clusters and later nitrogenase was shown to contain a distinctive MoFe7S9 active site. The development of metal carbonyl compounds 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.

The CO ligand can add to the polynuclear complex in three 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 (ReundefinedCl3)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 Re3Cl123- 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 MxMo6X8 such as PbundefinedMo6S8 form a Chevrel phase which exhibit superconductivity at low temperatures.

Zintl clusters
Zintl compounds represent a separate class of metal clusters. Historically, they were generated by reduction of metalloids with a solution of sodium in liquid ammonia. Examples of Zintl anions are Bi33-, Sn94-, [Pb7]4- and [Sb7]3-. These anions do not require ligands and are called naked clusters but are unstable and their isolation requires the use of cryptate complexes of the alkali metal cation. The structure of the Pb102- anion is that of a capped square antiprism. 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 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@Pb102- (the cage contains one nickel atom).

The icosahedral tin cluster Sn122- or stannaspherene anion is another closed shell structure observed (but not isolated) with photoelectron spectroscopy. 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.

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

Gas-phase clusters
Unstable clusters can also be observed in the gas-phase by means of mass spectroscopy even though they may be thermodynamically unstable and aggregate easily upon condensation. Such naked clusters, i.e. those that are not stabilized by ligands, are often produced by laser induced evaporation - or ablation - of a bulk metal or metal-containing compound. Typically, this approach produces a broad distribution of size distributions. Their electronic structures can be interrogated by techniques such as photoelectron spectroscopy. Their properties (Reactivity, Ionization potential, HOMO-LUMO-gap) often show a pronounced size dependence. Examples of such clusters are certain aluminium clusters as superatoms and certain gold clusters. Certain metal clusters are considered to exhibit metal aromaticity.

Carbon and Boron clusters
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.

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 a 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.