chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

, a polyoxometalate (abbreviated POM) is a

polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. The term molecule may or may no ...

, usually an

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

, that consists of three or more

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

oxyanions linked together by shared

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

atoms to form closed 3-dimensional frameworks. The metal atoms are usually

group 6 Group 6 may refer to: *Group 6 element, chemical element classification *Group 6 (racing) Group 6 was the official designation applied by the FIA to two motor racing classifications, the Prototype-Sports Car category from 1966 to 1971 and the Two ...

(Mo, W) or less commonly group 5 ( V, Nb, Ta) transition metals and Tc in their high

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

s. Polyoxometalates are often colorless, orange or red

diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...

s. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and

oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

, and

heteropolymetalate In chemistry, the heteropolymetalates are a subset of the polyoxometalates, which consist of three or more transition metal oxyanions linked together by shared oxygen atoms to form a closed 3-dimensional molecular framework. In contrast to isop ...

s, composed of one metal, oxide, and a main group oxyanion (

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is a ...

, etc.). Many exceptions to these general statements exist.

Formation

The oxides of d⁰ metals such as , , dissolve at high pH to give orthometalates, , , . For and , the nature of the dissolved species at high pH is less clear, but these oxides also form polyoxometalates. As the pH is lowered, orthometalates

protonate In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry aci ...

to give oxide–hydroxide compounds such as and . These species condense via the process called

olation In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important for understanding the relationship between metal aquo complexes and metal oxides, which are represented by man ...

. The replacement of terminal M=O bonds, which in fact have triple bond character, is compensated by the increase in coordination number. The nonobservation of polyoxochromate cages is rationalized by the small radius of Cr(VI), which may not accommodate octahedral coordination geometry. Condensation of the species entails loss of water and the formation of linkages. The stoichiometry for hexamolybdate is shown: :6 MoO4^2- + 10 HCl -> o6O192- + 10 Cl- + 5 H2O An abbreviated condensation sequence illustrated with vanadates is: :4 VO4^3- + 8 H+ -> V4O12^4- + 4 H2O :5 V4O12^4- + 12 H+ -> 2 V10O26(OH)2^4- + 4 H2O When such acidifications are conducted in the presence of

, heteropolymetalate result. For example, the phosphotungstate anion consists of a framework of twelve

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

tungsten oxyanions surrounding a central

group.

History

Ammonium phosphomolybdate Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula (NH4)3PMo12O40. The salt contains the phosphomolybdate anion, a well known heteropolymetalate of the Keggin structural class. Synthesis Ammonium ...

, anion, was reported in 1826. The isostructural phosphotungstate anion was characterized by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

1934. This structure is called the

Keggin structure Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α-Keggin anions, which have a general formula of , where X is the heteroatom (most commonly are pentavalent phosphorus PV, tetravalent silico ...

after its discoverer. The 1970's witnessed the introduction of

quaternary ammonium salt In chemistry, quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure , R being an alkyl group or an aryl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cation ...

s of POMs. This innovation enabled systematic study without the complications of hydrolysis and acid/base reactions. The introduction of ¹⁷O

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fi ...

allowed the structural characterization of POMs in solution. Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.

Structure and bonding

The typical framework building blocks are polyhedral units, with 6-coordinate metal centres. Usually, these units share edges and/or vertices. The coordination number of the oxide ligands varies according to their location in the cage. Surface oxides tend to be terminal or doubly bridging

oxo ligand A transition metal oxo complex is a coordination complex containing an oxo ligand. Formally O2-, an oxo ligand can be bound to one or more metal centers, i.e. it can exist as a terminal or (most commonly) as bridging ligands (Fig. 1). Oxo ligan ...

s. Interior oxides are typically triply bridging or even octahedral. POMs are sometimes viewed as soluble fragments of

metal oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

s. Recurring structural motifs allow POMs to be classified. ''Iso''-polyoxometalates (isopolyanions) feature octahedral metal centers. The

s form distinct structures because the main group center is usually tetrahedral. The Lindqvist and Keggin structures are common motifs for iso- and heteropolyanions, respectively. Polyoxometalates typically exhibit

coordinate In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sign ...

metal-oxo bonds of different multiplicity and strength. In a typical POM such as the Keggin structure , each addenda center connects to single terminal oxo ligand, four bridging µ₂-O ligands and one bridging µ₃-O deriving from the central heterogroup. Metal–metal bonds in polyoxometalates are normally absent and owing to this property, F. Albert Cotton opposed to consider polyoxometalates as form of cluster materials. However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.

Polymolybdates and tungstates

The polymolybdates and polytungstates are derived, formally at least, from the dianionic O₄sup>2- precursors. The most common units for polymolybdates and polyoxotungstates are the octahedral centers, sometimes slightly distorted. Some polymolybdates contain pentagonal bipyramidal units. These building blocks are found in the molybdenum blues, which are mixed valence compounds.

Polyoxotechnetate

Polyoxotechnetates form only in strongly acidic conditions, such as in or trifluoromethanesulfonic acid solutions. The first empirically isolated polyoxotechnetate was the red . It contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as the

hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid ...

salt by concentrating an solution. Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure.

Polyoxotantalates, niobates, and vanadates

The polyniobates, polytantalates, and vanadates are derived, formally at least, from highly charged O₄sup>3- precursors. For Nb and Ta, most common members are (M = Nb, Ta), which adopt the Lindqvist structure. These octaanions form in strongly basic conditions from alkali melts of the extended metal oxides (M₂O₅), or in the case of Nb even from mixtures of niobic acid and alkali metal hydroxides in aqueous solution. The hexatantalate can also be prepared by condensation of peroxotantalate in alkaline media. These polyoxometalates display an anomalous aqueous solubility trend of their alkali metal salts inasmuch as their Cs⁺ and Rb⁺ salts are more soluble than their Na⁺ and Li⁺ salts. The opposite trend is observed in

POMs. The decametalates with the formula (M = Nb, Ta) are isostructural with decavanadate. They are formed exclusively by edge-sharing octahedra (the structure of decatungstate comprises edge-sharing and corner-sharing tungstate octahedra).

Heteroatoms

Heteroatoms aside from the transition metal are a defining feature of

s. Many different elements can serve as heteroatoms but most common are , , and .

Giant structures

Polyoxomolybdates include the wheel-shaped molybdenum blue anions and spherical keplerates. The cluster consists of more than 700 atoms and is the size of a small protein. The anion is in the form of a tire (the cavity has a diameter of more than 20 Å) and an extremely large inner and outer surface. The incorporation of

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yt ...

ions in molybdenum blues is particularly intriguing.

Lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yt ...

s can behave like Lewis acids and perform catalytic properties.

-containing polyoxometalates show chemoselectivity and are also able to form inorganic–organic adducts, which can be exploited in

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

recognition.

Oxoalkoxometalates

Oxoalkoxometalates are clusters that contain both oxide and alkoxide ligands. Typically they lack terminal oxo ligands. Examples include the dodecatitanate Ti₁₂O₁₆(OPri)₁₆ (where OPri stands for an

alkoxy In chemistry, the alkoxy group is an alkyl group which is singularly bonded to oxygen; thus . The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the organic compound ethyl phenyl ether (, also ...

group), the iron oxoalkoxometalates and iron and copper Keggin ions.

Sulfido, imido, and other ''O''-replaced oxometalates

The terminal oxide centers of polyoxometalate framework can in certain cases be replaced with other ligands, such as S²⁻, Br⁻, and NR²⁻. Sulfur-substituted POMs are called ''polyoxothiometalates''. Other ligands replacing the oxide ions have also been demonstrated, such as nitrosyl and

groups. Polyfluoroxometalate are yet another class of O-replaced oxometalates.

Other

Numerous hybrid organic–inorganic materials that contain POM cores, Illustrative of the diverse structures of POM is the ion , which has face-shared octahedra with Mo atoms at the vertices of an icosahedron).

Use and aspirational applications

Oxidation catalysts

POMs are employed as commercial catalysts for oxidation of organic compounds. Efforts continue to extend this theme. POM-based aerobic oxidations have been promoted as alternatives to

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

-based wood pulp bleaching processes, a method of decontaminating water, and a method to catalytically produce formic acid from biomass ( OxFA process). Polyoxometalates have been shown to catalyse

water splitting Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: :2 H2O → 2 H2 + O2 Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy, base ...

Molecular electronics

Some POMs exhibit unusual magnetic properties, which has prompted visions of many applications. One example is storage devices called

qubit In quantum computing, a qubit () or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, ...

s. non-volatile (permanent) storage components, also known as

flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...

devices.

Drugs

Potential

antitumor Cancer can be treated by surgery, chemotherapy, radiation therapy, hormonal therapy, targeted therapy (including immunotherapy such as monoclonal antibody therapy) and synthetic lethality, most commonly as a series of separate treatments ...

and

antiviral Antiviral drugs are a class of medication used for treating viral infections. Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do n ...

drugs. The Anderson-type polyoxomolybdates and heptamolybdates exhibit activity for suppressing the growth of some tumors. In the case of (NH₃Pr)₆ o₇O₂₄ activity appears related to its redox properties. The Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's disease. antibacterial and antiviral uses.