In
chemistryChemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...
, a
cluster is an ensemble of bound
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s intermediate in size between a
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
and a bulk
solidMatter is generally found in three different forms: solid, liquid, and gas . The solid state of matter is characterized by a distinct structural rigidity and resistance to deformation . Most solids have high values both of Young's modulus and of the shear modulus of elasticity...
. Clusters exist of diverse stoichiometries and nuclearities. For example,
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite....
atoms form
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
and
boraneIn chemistry, a borane is a chemical compound of boron and hydrogen. The boranes comprise a large group of compounds with the generic formulae of BxHy. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently...
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 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 .
The main cluster types are "naked" clusters (without stabilizing
ligandIn chemistry, a ligand is either an atom, ion, or molecule that binds to a central metal-atom to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The metal-ligand bonding ranges from covalent...
s) and those with ligands. Typical ligands that stabilize clusters include
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
,
halideA halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s,
isocyanideAn isocyanide is an organic compound with the functional group R-N≡C. The CN functionality is connected to the organic fragment via the nitrogen atom, not via carbon as is found in the isomeric nitriles, which have the connectivity R-C≡N. Hence the prefix iso...
s,
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s, and
hydrideHydride is the name given to the negative ion of hydrogen, H−. Practically, the term hydride has two distinct but overlapping meanings. In the chemical vernacular the term hydride refers to a hydrogen atom that formally reacts as a hydrogen anion under common conditions as well as...
s.
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, but no industrial applications exist. The clusters
Ru3(CO)12Triruthenium dodecarbonyl is the chemical compound with the formula Ru312. This orange-colored metal carbonyl cluster is a precursor to other organoruthenium compounds.-Structure and synthesis:...
and
Ir4(CO)12Tetrairidium dodecacarbonyl is the chemical compound with the formula Ir412. This tetrahedral cluster is the most common and most stable "binary" carbonyl of iridium. This air-stable species is only poorly soluble in organic solvents. It has been used to prepare bimetallic...
catalyse the
Water gas shift reactionThe water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction...
, also catalyzed by iron oxide, and Rh
6(CO)
16 catalyzes the conversion of carbon monoxide into hydrocarbons, reminiscent of the
Fischer-Tropsch processThe Fischer–Tropsch process is a catalyzed chemical reaction in which synthesis gas, a mixture of carbon monoxide and hydrogen, is converted into liquid hydrocarbons of various forms. The most common catalysts are based on iron and cobalt, although nickel and ruthenium have also been used...
, 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
nitrogenaseNitrogenase is the enzyme used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes which accomplishes this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
. CO is oxidized to CO
2 by the Fe-Ni-S cluster carbon monoxide dehydrogenase.
HydrogenaseA hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
s rely on Fe
2 and NiFe clusters.
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
refractory metalRefractory metals are a class of metals that are extraordinarily resistant to heat and wear. Refractory metals are said to be poorly resistant to oxidation and corrosion. The definition which elements belong to this group differs...
atoms. In general metal centers with large d-orbitals form stable clusters because of favorable overlap of valence orbitals. Thus, metals with a low
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
for the later metals and mid-oxidation states for the early metals tend to form stable clusters. Polynuclear
metal carbonylMetal carbonyls are coordination complexes of transition metals with carbon monoxide. These complexes may be homoleptic, i.e. contain only CO ligands, such as nickel carbonyl , but more commonly metal carbonyls contain a mix of ligands, such as Re
3Cl...
s are generally found in late
transition metalThe term transition metal has two possible meanings:*In the past it referred to any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table...
s with low formal oxidation states.
The
polyhedral skeletal electron pair theoryIn chemistry the polyhedral skeletal electron pair theory provides electron counting rules used to predict the structure of electron deficient clusters. They were originally formulated by K. Wade and further developed by Mingos and others, and are sometimes known as Wade's Rules or Wade / Mingos...
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
IndiaIndia, officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
already in the 12th century. The existence of a
mercuryMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
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)9Diiron nonacarbonyl is an inorganic compound with the formula Fe29. This metal carbonyl is an important reagent in organometallic chemistry and of occasional use in organic synthesis. It is a more reactive source of Fe than Fe5 and less dangerous to handle because...
and
Fe3(CO)12Triiron dodecarbonyl is the chemical compound with the formula Fe312. It was one of the first metal carbonyl clusters synthesized. It is a more reactive source of iron than is iron pentacarbonyl.-General properties:...
. Rundle and Dahl discovered that
Mn2(CO)10Dimanganese decacarbonyl is the chemical compound with the formula Mn210. This metal carbonyl is an important reagent in the organometallic chemistry of manganese.-Synthesis:...
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 [Rh
13(CO)
24H
3]
2-.
Transition metal halide clusters
Linus PaulingLinus Carl Pauling was an American chemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists in any field of the 20th century. Pauling was among the first scientists to work in the fields of quantum...
showed that "
MoCl2Molybdenum dichloride describes chemical compounds with the empirical formula MoCl2. At least two forms are known, and both have attracted much attention from academic researchers because of the unexpected structures seen for these compounds and the fact that they give rise to hundreds...
" consisted of Mo
6 octahedra.
F. Albert CottonFrank Albert Cotton was the W.T. Doherty-Welch Foundation Chair and Distinguished Professor of Chemistry at Texas A&M University. He authored over 1700 scientific articles. Cotton was recognized for his research on the chemistry of the transition metals.-Education:Frank Albert Cotton was born on...
established that "ReCl
3" in fact features subunits of the cluster Re
3Cl
9, which could be converted to a host of adducts without breaking the Re-Re bonds. Because this compound is diamagnetic and not paramagnetic the
rheniumRhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust...
bonds are
double bondA double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
s and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
a Re
3Cl
123- complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W
4(OCH
3)
12 with
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite...
single bonds and molybdenum chloride (
Mo6Cl8)Cl4Molybdenum dichloride describes chemical compounds with the empirical formula MoCl2. At least two forms are known, and both have attracted much attention from academic researchers because of the unexpected structures seen for these compounds and the fact that they give rise to hundreds...
is a hexanuclear
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The free element, which is a silvery metal, has the sixth-highest melting point of any element. It readily forms hard, stable carbides, and for this reason it is often used in high-strength steel alloys...
compound and an example of an
octahedral clusterOctahedral clusters are inorganic or organometallic cluster compounds composed of six metals in an octahedral array. One important class of octahedral clusters are chalcohalide clusters of the type M6X8L6 where M is a metal usually of group 6 or group 7, X is a...
. A related group of clusters with the general formula M
xMo
6X
8 such as Pb
Mo
6S
8 form a Chevrel phase, which exhibit
superconductivitySuperconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field . It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral...
at low temperatures. The eclipsed structure of potassium octachlorodirhenate(III), K
2Re
2Cl
8 was explained by invoking
Quadruple bondA quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum...
ing. This discovery led to a broad range of derivatives including
di-tungsten tetra(hpp)Di-tungsten tetra is the name of the coordination compound with the formula W24. This material consists of a pair of tungsten centers linked by the conjugate base of four hexahydropyrimidopyrimidine ligands...
, 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 StockAlfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
and his successors who popularized the use of vacuum-lines for the manipulation of these often volatile, air-sensitive materials. Clusters of boron are
boraneIn chemistry, a borane is a chemical compound of boron and hydrogen. The boranes comprise a large group of compounds with the generic formulae of BxHy. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently...
s such as
pentaboranePentaborane, also called pentaboron nonahydride, stable pentaborane, or pentaborane , is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by both the U.S. and Russian armed services, a so-called "exotic fuel"...
and
decaboraneDecaborane, also called decaborane, is the borane with the chemical formula B10H14. This white crystalline compound is one of the principal boron hydride clusters, both as a reference structure and as a precursor to other boron hydrides.-Handling properties and...
. Composite clusters containing CH and BH vertices are
carboraneA carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...
s.
Fe-S clusters in biology
In the 1970s,
ferredoxinFerredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
was demonstrated to contain Fe
4S
4 clusters and later
nitrogenaseNitrogenase is the enzyme used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes which accomplishes this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
was shown to contain a distinctive MoFe
7S
9 active site. With the development of
bioinorganic chemistryBioinorganic chemistry is a field that examines the role of metals in biology. Bioinorganic chemistry includes the study of both natural phenomena such as the behavior of metalloproteins as well artificially introduced metals, including those that are non-essential, in medicine and toxicology...
, a variety of synthetic analogues of these clusters have been described.
Zintl clusters
Zintl compoundsIn chemistry a Zintl phase is the product of a reaction between
*group 1 or group 2 and*post transition metals or metalloids from group 13, 14, 15 or 16....
feature naked anionic clusters that are generated by reduction of heavy main group
p elements, mostly metals or semimetals, with alkali metals, often as a solution in anhydrous liquid
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
or ethylenediamine. Examples of Zintl anions are [Bi
3]
3−, [Sn
9]
4−, [Pb
7]
4−, and [Sb
7]
3−. Although these species are called "naked clusters," they are usually strongly associated with alkali metal cations. Some examples have been isolated using cryptate complexes of the alkali metal cation, e.g., [Pb
10]
2− anion, which features a capped
square antiprismIn geometry, the square antiprism is the second in an infinite set of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps...
atic shape. 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 K
4Pb
9 by Au
+ in PPh
3AuCl (by reaction of tetrachloroauric acid and
triphenylphosphineTriphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at...
) in
ethylene diamineEthylenediamine is the organic compound with the formula C2H42. This colorless liquid with an ammonia-like odor is a strongly basic amine...
with 2.2.2-crypt. This type of cluster was already known as is the endohedral Ni@Pb
102− (the cage contains one
nickelNickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
atom). The icosahedral
tinTin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead, like the two possible oxidation states +2 and +4...
cluster Sn
122− or
stannaspherene anion is another closed shell structure observed (but not isolated) with photoelectron spectroscopy. With an internal diameter of 6.1
AngstromThe ångström or angstrom is an internationally recognized unit of length equal to 0.1 nanometre or 1 metres. It is named after Anders Jonas Ångström...
it is of comparable size to
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
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
mass spectrometryMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
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, while
infrared multiphoton dissociationInfrared multiphoton dissociation is a technique used in mass spectrometry to fragment molecules in the gas phase usually for structural analysis of the original molecule. -How it works:...
spectroscopy is more probing the clusters geometry. Their properties (
ReactivityReactivity refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. In pure compounds, reactivity is regulated by the physical properties of the sample. For instance, grinding a sample to a higher specific surface area increases its reactivity. In impure...
,
Ionization potentialThe term ionization energy is most commonly used to refer to the energy required to remove the outermost electron in the atom or molecule when the gas atom or molecule is isolated in free space and is in its ground electronic state...
,
HOMO-Science:*Homo , the genus including modern humans and closely related species such as Neanderthals*Highest Occupied Molecular Orbital, in chemistry: see HOMO/LUMO-Organisations:...
-
LUMOLumo is a 2007 documentary film about twenty-year-old Lumo Sinai, a woman who fell victim to "Africa's First World War." While returning home one day, Lumo and another woman were gang-raped by a group of soldiers fighting for control of the Democratic Republic of the Congo during the 1994 Rwandan...
-gap) often show a pronounced size dependence. Examples of such clusters are certain aluminium clusters as superatoms and certain
gold clusterGold clusters in cluster chemistry are gold-derived materials that can either be discrete molecules or larger colloidal particles. Both types are describes as nanoparticles, with diameters of less than one micrometer.-Discrete gold clusters:...
s. Certain metal clusters are considered to exhibit
metal aromaticityIn metal aromaticity the concept of aromaticity found in many hydrocarbons is extended to metals. The first experimental evidence for the existence of aromaticity in metals was found in aluminium cluster compounds of the type MAl4- where M stands for lithium, sodium or copper...
. In some cases, the results of laser ablation experiments are translated to isolated compounds, and the premier cases are the clusters of carbon called the
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
s, notably clusters with the formula C
60, C
70, and C
84. The fullerene sphere can be filled with small molecules in
Endohedral fullerenesEndohedral fullerenes are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex was synthesized in 1985 called La@C60. The @ sign in the name reflects the notion of a small molecule trapped inside a shell...
.
Extended metal atom chains
Extended metal atom chain complexesExtended metal atom chains are molecules that consist of a linear string of directly bonded metal atoms, surrounded by organic ligands. These compounds represent the smallest molecular wires...
(
EMAC) are a novel topic in academic research. They are comprised of linear chains of metal atoms stabilized with
ligandIn chemistry, a ligand is either an atom, ion, or molecule that binds to a central metal-atom to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The metal-ligand bonding ranges from covalent...
s. EMACS are known based on
nickelNickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
(with 9 atoms),
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
and
cobaltCobalt is a hard, lustrous, gray metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times for making jewelry and paints, and miners have long used the name kobold ore for some minerals, the free metallic cobalt was...
(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 an EMAC was obtained that consisted of 9 chromium atoms in a linear array with 4 ligands (based on an
oligoAn oligonucleotide is a short nucleic acid polymer, typically with twenty or fewer bases. Although they can be formed by bond cleavage of longer segments, they are now more commonly synthesized by polymerizing individual nucleotide precursors...
pyridinePyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
) wrapped around it. In it the chromium chain contains 4
quadruple bondA quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum...
s.
External links
- http://cluster-science.net - scientific community portal for clusters, fullerenes, nanotubes, nanostructures, and similar small systems
See also
- Cluster (physics)
In physics, the term clusters denotes small, multiatom particles. As a rule of thumb, any particle of somewhere between 3 and 3x107 atoms is considered a cluster. Two-atom particles are sometimes considered clusters as well...
- Water molecules form clusters as well: see water cluster
In chemistry a water cluster is a discrete hydrogen bonded assembly or cluster of molecules of water. These clusters have been found experimentally or predicted in silico in various forms of water; in ice, in crystal lattices and in bulk liquid water, the simplest one being the water dimer...
s