Cluster chemistry
Encyclopedia
In chemistry
, a cluster is an ensemble of bound atom
s 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 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 prefixed terms “nuclear” and “metallic” are used and imply different meanings. For example, polynuclear refers to a cluster with more than one metal atom, regardless of the elemental identities. Heteronuclear refers to a cluster with at least two different metal elements.
The main cluster types are "naked" clusters (without stabilizing ligand
s) and those with ligands. Typical ligands that stabilize clusters include carbon monoxide
, halide
s, isocyanide
s, alkene
s, and hydride
s.
and Ir4(CO)12
catalyze the Water gas shift reaction
, also catalyzed by iron oxide
, and Rh6(CO)16 catalyzes the conversion of carbon monoxide into hydrocarbons, reminiscent of the Fischer-Tropsch process
, also catalyzed by iron compounds.
Although having no role in industrial catalysis, clusters are common catalysts 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. Hydrogenase
s rely on Fe2 and NiFe clusters.
Metal clusters here refer to polyatomic assemblies having at least one metal-metal bond, such as Fe-Ni Clusters
. Cluster catalysis can be separated into three types, including (1) homogeneous, relatively low-nuclearity clusters that are soluble in solvents, (2) heterogeneous, insoluble cluster networks or colloids that can be suspended in a solvent, and (3) heterogeneous, relatively high-nuclearity nanoparticles that can be either clean surface or surface-ligated, supported or free.
The participation of clusters in a catalytic reaction can be evaluated using the following set of criteria:
Metal clusters have several properties that suggest that they may prove as useful catalysts. The absence of large bulk phases leads to a high surface-to-volume ratio, which is advantageous in any catalyst application as this maximizes the reaction rate per unit amount of catalyst material, which also minimizes cost. Catalysts are characterized by active adsorption and reaction sites that lower an activation barrier to a particular chemical transformation. Metal clusters contain these sites as a rule, as surface metal atoms are inherently undercoordinated, leading to a low adsorption energy barrier. In general, as the number of atoms in a metal particle decrease, their coordination number decreases, and significantly so in particles having less than 100 atoms. This is illustrated by the figure at right, which shows dispersion (ratio of undercoordinated surface atoms to total atoms) versus number of metal atoms per particle for ideal isocahedral metal clusters.
Metal clusters are sometimes characterized by a high degree of fluxionality of surface ligands and adsorbates associated with a low energy barrier to rearrangement of these species on the surface. The rearrangement of ligands on a cluster exterior is related to the diffusion of adsorbates on solid metal surfaces. Interconversion ligands between terminal, double-, and triply bridging sites is often facile. It has further been found that metal atoms themselves can easily migrate in or break their bonds with the cluster structure.
In some cases, a metal cluster must be “activated” for catalysis by substitution of one or more ligands, such as acetonitrile. For example, Os3(CO)12 will have one active site after thermolysis and the dissociation of a single carbonyl group. Os3(CO)10(CH3CN)2 will have two active sites.
Electronic effects can be illustrated by bimetallic catalysts Fe-Pt and Fe-Rh, where Pt and Rh each contribute a different electronic character to the cluster. Rh atoms can coordinate to the carbon atom of a carbonyl group and an alkyl group simultaneously, whereas Pt coordinates to the carbonyl and a hydrogen atom. The result is that Fe-Pt produces methanol and Fe-Rh produces alkanols. These reactions are illustrated below:
(DFT). An example of the former is empirical packing energy calculations, where only interactions between adjacent atoms are considered. The packing potential energy can be expressed as follows:
where index i refers to all atoms of a reference molecule in the cluster lattice and index j refers to atoms in surrounding molecules according to crystal symmetry, and A, B, and C are parameters. The advantage of such methods is ease of computation, however accuracy is dependant on the particular assumptions made. DFT has been used more recently to study a wide variety of properties of metal clusters. Its advantages are being a first-principles approach without need of parameters and the ability to study clusters without ligands of a definitive size. However the fundamental form of the energy functional is only approximately known, and unlike other methods there is no hierarchy of approximations which allow a systematic optimization of results.
Tight bonding molecular dynamics has been used to study bond lengths, bond energies, and magnetic properties of metal clusters, however this method is less effective for clusters with less than 10-20 metal atoms due to a larger influence of approximation errors for small clusters. There are limitations on the other extreme as well that exist with any computation method, that the approach to bulk-like properties is difficult to capture because at these cluster sizes the cluster model becomes increasingly complex.
atoms. In general metal centers with extended d-orbitals form stable clusters because of favorable overlap of valence orbitals. Thus, metals with a low oxidation state
for the later metals and mid-oxidation states for the early metals tend to form stable clusters. Polynuclear metal carbonyl
s are generally found in late transition metal
s with low formal 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.
already in the 12th century. The existence of a mercury
to mercury bond in this compound was established in beginning of the 20th century.
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 1970s, Paolo Chini demonstrated that very large clusters could be prepared from the platinum metals, one example being [Rh13(CO)24H3]2-.
showed that "MoCl2
" consisted of Mo6 octahedra. F. Albert Cotton
established that "ReCl3" in fact features subunits of 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 bond
s 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 related group of clusters with the general formula MxMo6X8 such as PbMo6S8 form a Chevrel phase, which exhibit superconductivity
at low temperatures. The eclipsed structure of potassium octachlorodirhenate(III), K2Re2Cl8 was explained by invoking Quadruple bond
ing. This discovery led to a broad range of derivatives including di-tungsten tetra(hpp)
, the current (2007) record holder low ionization energy
.
and his successors who popularized the use of vacuum-lines for the manipulation of these often volatile, air-sensitive materials. Clusters of boron are borane
s such as pentaborane
and decaborane
. Composite clusters containing CH and BH vertices are carborane
s.
was demonstrated to contain Fe4S4 clusters and later nitrogenase
was shown to contain a distinctive MoFe7S9 active site. With the development of bioinorganic chemistry
, a variety of synthetic analogues of these clusters have been described.
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 ammonia
or ethylenediamine. Examples of Zintl anions are [Bi3]3−, [Sn9]4−, [Pb7]4−, and [Sb7]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., [Pb10]2− anion, which features a capped square antiprism
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 K4Pb9 by Au+ in PPh3AuCl (by reaction of tetrachloroauric acid 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 in the same manner as endohedral fullerenes.
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 dissociation
spectroscopy is more probing the clusters geometry. 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 cluster
s. Certain metal clusters are considered to exhibit metal aromaticity
. 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 fullerene
s, notably clusters with the formula C60, C70, and C84. The fullerene sphere can be filled with small molecules in Endohedral fullerenes
.
(EMAC) are a novel topic in academic research. An EMAC is composed of linear chains of metal atoms stabilized with ligand
s. 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 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 bond
s.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a cluster is an ensemble of bound atom
Atom
The atom is a basic unit of matter that consists 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 molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
and a bulk solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon
Carbon
Carbon 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 boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
atoms form fullerene
Fullerene
A 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 they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
and borane
Borane
In 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. The parent member BH3 is called...
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 prefixed terms “nuclear” and “metallic” are used and imply different meanings. For example, polynuclear refers to a cluster with more than one metal atom, regardless of the elemental identities. Heteronuclear refers to a cluster with at least two different metal elements.
The main cluster types are "naked" clusters (without stabilizing ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s) and those with ligands. Typical ligands that stabilize clusters include carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, halide
Halide
A 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, isocyanide
Isocyanide
An isocyanide is an organic compound with the functional group -N≡C. It is the isomer of the related cyanide , hence the prefix iso....
s, alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s, and hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
s.
Applications of clusters in catalysis
Metal cluster compounds, especially those having numbers of carbonyl ligands, have been evaluated as catalysts for a wide range of reactions, especially related to carbon monoxide utilization, but no industrial applications exist. The clusters Ru3(CO)12Triruthenium dodecacarbonyl
Triruthenium dodecacarbonyl 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)12
Tetrairidium dodecacarbonyl
Tetrairidium 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 clusters and catalysts,...
catalyze the Water gas shift reaction
Water gas shift reaction
The water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is...
, also catalyzed by iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
, and Rh6(CO)16 catalyzes the conversion of carbon monoxide into hydrocarbons, reminiscent of the Fischer-Tropsch process
Fischer-Tropsch process
The Fischer–Tropsch process is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a petroleum substitute, typically from coal, natural gas, or biomass for use as synthetic...
, also catalyzed by iron compounds.
Although having no role in industrial catalysis, clusters are common catalysts 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
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
. CO is oxidized to CO2 by the Fe-Ni-S cluster carbon monoxide dehydrogenase. Hydrogenase
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
s rely on Fe2 and NiFe clusters.
Metal clusters here refer to polyatomic assemblies having at least one metal-metal bond, such as Fe-Ni Clusters
Fe-Ni Clusters
thumb|350px|right|Figure 1: Closed Triangulated polyhedra. Tetrahedron , Trigonal bipyramid . Octahedron . Pentagonal bipyramid . Capped octahedron...
. Cluster catalysis can be separated into three types, including (1) homogeneous, relatively low-nuclearity clusters that are soluble in solvents, (2) heterogeneous, insoluble cluster networks or colloids that can be suspended in a solvent, and (3) heterogeneous, relatively high-nuclearity nanoparticles that can be either clean surface or surface-ligated, supported or free.
Definition of cluster catalysis
Although synthetic clusters have few practical applications in catalysis, the promise of their applications has led to extensive academic research. Catalysis as a unique type of catalysis can be defined with various degrees of rigor. The strictest definition requires that at least two active sites on two different metal atoms be involved in the reaction cycle. Some define cluster catalysis to include clusters that have only one active site on one metal atom. The definition can be further relaxed to include clusters that are completely intact during at least one reaction step, and can be fragmented in all others.The participation of clusters in a catalytic reaction can be evaluated using the following set of criteria:
- Turnover frequency increases with increasing cluster concentration
- Product selectivities are different when using a polynuclear cluster than when using a mononuclear complex
- Combination of 2 or more metals in a cluster significantly enhances the reaction rate, changes the product selectivity, or allows a reaction to proceed that was not catalyzed when using a homonuclear cluster (this further indicates mixed metal catalysis)
- Modification of a cluster or the reaction conditions in a way that promotes metal-metal bond formation results in an increased reaction rate
- Catalytic asymmetric inductionAsymmetric inductionAsymmetric induction in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment...
occurs with a chiral metal cluster in which the asymmetry is an inherent property of the cluster
Metal clusters have several properties that suggest that they may prove as useful catalysts. The absence of large bulk phases leads to a high surface-to-volume ratio, which is advantageous in any catalyst application as this maximizes the reaction rate per unit amount of catalyst material, which also minimizes cost. Catalysts are characterized by active adsorption and reaction sites that lower an activation barrier to a particular chemical transformation. Metal clusters contain these sites as a rule, as surface metal atoms are inherently undercoordinated, leading to a low adsorption energy barrier. In general, as the number of atoms in a metal particle decrease, their coordination number decreases, and significantly so in particles having less than 100 atoms. This is illustrated by the figure at right, which shows dispersion (ratio of undercoordinated surface atoms to total atoms) versus number of metal atoms per particle for ideal isocahedral metal clusters.
Metal clusters are sometimes characterized by a high degree of fluxionality of surface ligands and adsorbates associated with a low energy barrier to rearrangement of these species on the surface. The rearrangement of ligands on a cluster exterior is related to the diffusion of adsorbates on solid metal surfaces. Interconversion ligands between terminal, double-, and triply bridging sites is often facile. It has further been found that metal atoms themselves can easily migrate in or break their bonds with the cluster structure.
Examples of reactions catalysed by metal carbonyl clusters
Although metal carbonyl clusters are rarely used, they have been subjected to many studies aimed at demonstrating their reactivity. Some of these examples include the following| Reaction | Core Metals | Catalyst | Reference |
|---|---|---|---|
| Alkene hydroformylation | Mo-Rh | Mo2RhCp3(CO)5 | |
| Alkene hydroformylation | Rh | Rh4(CO)10+x(PPh3)2-x (x=0,2) | |
| CO hydrogenation | Ru-Os | H2RuOs3(CO)13 | |
| CO hydrogenation | Ru-Co | RuCo2(CO)11 | |
| CO hydrogenation | Ir | Ir4(CO)12 | |
| CO hydrogenation | Fe | Fe3(CO)12 | |
| Alkene hydrogenation | Os-Ni | H3Os3NiCp(CO)9 | |
| Alkene hydrogenation | Ni | Ni2+xCp2+x(CO)2 (x=0,1) | |
| Alkyne hydrogenation | Os-Ni | Os3Ni3Cp3(CO)9 | |
| Hydrogenation of aromatics | Ni | Ni2+xCp2+x(CO)2 (x=0,1) | |
| Acetaldehyde hydrogenation | Ni | Ni4(Me3CNC)7 | |
| Alkene isomerization | V-Cr | VCrCp3(CO)3 | |
| Hydrocarbon isomerization | Fe-Pt | Fe2Pt(CO)6(NO)2(Me3CNC)2 | |
| Butane hydrogenolysis | Rh-Ir | Rh3+xIr3-x(CO)16 (x=0,1,2) | |
| Methanol hydrocarbonylation | Ru-Co | Ru2Co2(CO)13 | |
| Hydrodesulfurization | Mo-Fe | Mo2Fe2S2Cp2(CO)8 | |
| CO and CO2 methanation | Ru-Co | HRuCo3(CO)12 | |
| Ammonia synthesis | Ru-Ni | H3Ru3NiCp(CO)9 | |
Fischer-Tropsch catalysis
Species that are typical ligands for a metal cluster represent obvious reactant-catalyst combinations. For example, hydrogenation of CO (Fischer-Tropsch synthesis) can be catalyzed using several metal clusters, as shown in the table above. It has been proposed that coordination of CO to multiple metal sites weakens the triple-bond enough to allow hydrogenation. As in the industrially significant heterogeneous process, Fischer-Tropsch synthesis by clusters yields alkanes, alkenes, and various oxygenates. The selectivity is heavily influenced by the particular cluster used. For example, Ir4(CO)12 produces methanol, whereas Ru2Rh(CO)12 produces ethylene glycol. Selectivity is determined by several factors, including steric and electronic effects. Steric effects are the most important consideration in many cases, however electronic effects dominate in hydrogenation reactions where one adsorbate (hydrogen) is relatively small. Where steric effects are dominating, selectivity can be different depending on the arrangement or types of ligands.Clusters in the synthesis of thiacrown ethers
The cyclooligomerization of thiotanes illustrates the influence of steric effects on selectivity. The reaction is shown at right. The selectivity ratio S3(CH2)9/S6(CH2)18 is 6.0 for Os4(CO)12 and 1.5 for W(CO)6, rationalized by greater steric effects in the Os cluster, leading to a preference for the smaller ring product.In some cases, a metal cluster must be “activated” for catalysis by substitution of one or more ligands, such as acetonitrile. For example, Os3(CO)12 will have one active site after thermolysis and the dissociation of a single carbonyl group. Os3(CO)10(CH3CN)2 will have two active sites.
Electronic effects can be illustrated by bimetallic catalysts Fe-Pt and Fe-Rh, where Pt and Rh each contribute a different electronic character to the cluster. Rh atoms can coordinate to the carbon atom of a carbonyl group and an alkyl group simultaneously, whereas Pt coordinates to the carbonyl and a hydrogen atom. The result is that Fe-Pt produces methanol and Fe-Rh produces alkanols. These reactions are illustrated below:
Methods of study
Metal clusters have been studied using both experimental and computational methods. Single-crystal X-ray crystallography has been used for many years to determine the solid state structure of transition metal clusters prepared experimentally. Reactivity studies are also useful, especially in determining the catalytic properties of clusters.Computational studies
Computational studies have progressed from sum-of-energies calculations (incorporating Huckel theory-type approximations) to Density Functional TheoryDensity functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
(DFT). An example of the former is empirical packing energy calculations, where only interactions between adjacent atoms are considered. The packing potential energy can be expressed as follows:
where index i refers to all atoms of a reference molecule in the cluster lattice and index j refers to atoms in surrounding molecules according to crystal symmetry, and A, B, and C are parameters. The advantage of such methods is ease of computation, however accuracy is dependant on the particular assumptions made. DFT has been used more recently to study a wide variety of properties of metal clusters. Its advantages are being a first-principles approach without need of parameters and the ability to study clusters without ligands of a definitive size. However the fundamental form of the energy functional is only approximately known, and unlike other methods there is no hierarchy of approximations which allow a systematic optimization of results.
Tight bonding molecular dynamics has been used to study bond lengths, bond energies, and magnetic properties of metal clusters, however this method is less effective for clusters with less than 10-20 metal atoms due to a larger influence of approximation errors for small clusters. There are limitations on the other extreme as well that exist with any computation method, that the approach to bulk-like properties is difficult to capture because at these cluster sizes the cluster model becomes increasingly complex.
Electronic structure
Metal clusters are frequently composed of refractory metalRefractory metals
Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group differs...
atoms. In general metal centers with extended d-orbitals form stable clusters because of favorable overlap of valence orbitals. Thus, metals with a low oxidation state
Oxidation state
In 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 carbonyl
Metal carbonyl
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. These complexes may be homoleptic, that is containing only CO ligands, such as nickel carbonyl , but more commonly metal carbonyls contain a mix of ligands, such as Re3Cl...
s are generally found in late transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s with low formal oxidation states.
The polyhedral skeletal electron pair theory
Polyhedral skeletal electron pair theory
In chemistry the polyhedral skeletal electron pair theory provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by Kenneth Wade and were further developed by D. M. P. Mingos and...
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
India , 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 with over 1.2 billion people, and the most populous democracy in the world...
already in the 12th century. The existence of a mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
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
Diiron 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 it is nonvolatile...
and Fe3(CO)12
Triiron dodecacarbonyl
Triiron 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)10
Dimanganese decacarbonyl
Dimanganese 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 1970s, 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
Linus PaulingLinus Pauling
Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...
showed that "MoCl2
Molybdenum(II) chloride
Molybdenum 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 of...
" consisted of Mo6 octahedra. F. Albert Cotton
F. Albert Cotton
Frank 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 "ReCl3" in fact features subunits of 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
Rhenium
Rhenium 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. The free element has...
bonds are double bond
Double bond
A 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 acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
a Re3Cl123- complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W4(OCH3)12 with tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
single bonds and molybdenum chloride (Mo6Cl8)Cl4
Molybdenum(II) chloride
Molybdenum 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 of...
is a hexanuclear molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
compound and an example of an octahedral cluster
Octahedral cluster
Octahedral 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 ligand and more specifically an...
. A related group of clusters with the general formula MxMo6X8 such as PbMo6S8 form a Chevrel phase, which exhibit superconductivity
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
at low temperatures. The eclipsed structure of potassium octachlorodirhenate(III), K2Re2Cl8 was explained by invoking Quadruple bond
Quadruple bond
A 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(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
Ionization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
.
Boron hydrides
Contemporaneously with the development of metal cluster compounds, numerous boron hydrides were discovered by Alfred StockAlfred Stock
Alfred 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 borane
Borane
In 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. The parent member BH3 is called...
s such as pentaborane
Pentaborane
Pentaborane, also called pentaborane to distinguish it from B5H11, is an inorganic compound with the formula B5H9. It is one of the most common boron hydride clusters, although it is a highly reactive compound. Because of its high reactivity toward oxygen, it was once evaluated as rocket or jet fuel...
and decaborane
Decaborane
Decaborane, 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....
. Composite clusters containing CH and BH vertices are carborane
Carborane
A 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, ferredoxinFerredoxin
Ferredoxins 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 Fe4S4 clusters and later nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
was shown to contain a distinctive MoFe7S9 active site. With the development of bioinorganic chemistry
Bioinorganic chemistry
Bioinorganic 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 compoundsZintl phase
In 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 ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
or ethylenediamine. Examples of Zintl anions are [Bi3]3−, [Sn9]4−, [Pb7]4−, and [Sb7]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., [Pb10]2− anion, which features a capped square antiprism
Square antiprism
In 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 K4Pb9 by Au+ in PPh3AuCl (by reaction of tetrachloroauric acid and triphenylphosphine
Triphenylphosphine
Triphenylphosphine 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 room temperature...
) in ethylene diamine
Ethylene diamine
Ethylenediamine is the organic compound with the formula C2H42. This colorless liquid with an ammonia-like odor is a strongly basic amine. The liquid fumes upon contact with humid air...
with 2.2.2-crypt. This type of cluster was already known as is the endohedral Ni@Pb102− (the cage contains one nickel
Nickel
Nickel 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. Nickel belongs to the transition metals and is hard and ductile...
atom). The icosahedral tin
Tin
Tin 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 and has two possible oxidation states, +2 and the slightly more stable +4...
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
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
it is of comparable size to fullerene
Fullerene
A 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 they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
and should be capable of containing small atoms in the same manner as endohedral fullerenes.
Gas-phase clusters and fullerenes
Unstable clusters can also be observed in the gas-phase by means of mass spectrometryMass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
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 dissociation
Infrared multiphoton dissociation
Infrared 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 (Reactivity, Ionization potential
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
, HOMO
Homo
Homo may refer to:*the Greek prefix ὅμο-, meaning "the same"*the Latin for man, human being*Homo, the taxonomical genus including modern humans...
-LUMO
Lumo
Lumo 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 cluster
Gold cluster
Gold 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 aromaticity
Metal aromaticity
In 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 fullerene
Fullerene
A 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 they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
s, notably clusters with the formula C60, C70, and C84. The fullerene sphere can be filled with small molecules in Endohedral fullerenes
Endohedral fullerenes
Endohedral 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
Extended 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. An EMAC is composed of linear chains of metal atoms stabilized with ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. EMACs are known based on nickel
Nickel
Nickel 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. Nickel belongs to the transition metals and is hard and ductile...
(with 9 atoms), chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the 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 odorless, tasteless, and malleable...
and cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
(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 oligo
Oligonucleotide
An oligonucleotide is a short nucleic acid polymer, typically with fifty or fewer bases. Although they can be formed by bond cleavage of longer segments, they are now more commonly synthesized, in a sequence-specific manner, from individual nucleoside phosphoramidites...
pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
) wrapped around it. In it the chromium chain contains 4 quadruple bond
Quadruple bond
A 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)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 clusterWater clusterIn 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 2...
s