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A transition metal oxo complex is a
coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
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 ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually r ...
s (Fig. 1). Oxo ligands stabilize high oxidation states of a metal.Nugent, W. A., Mayer, J. M. "Metal-Ligand Multiple Bonds." John Wiley & Sons, New York, 1988. They are also found in several
metalloproteins Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains al ...
, for example in molybdenum cofactors and in many iron-containing enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is
potassium ferrate Potassium ferrate is the chemical compound with the formula . This purple salt is paramagnetic, and is a rare example of an iron(VI) compound. In most of its compounds, iron has the oxidation state +2 or +3 ( or ). Reflecting its high oxidation sta ...
(K2FeO4), which was likely prepared by Georg E. Stahl in 1702.


Reactivity


Olation and acid-base reactions

A common reaction exhibited by metal-oxo compounds is
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 m ...
, the condensation process that converts low molecular weight oxides to polymers with M-O-M linkages. Olation often begins with the deprotonation of a metal-hydroxo complex. It is the basis for mineralization and the precipitation of metal oxides. For the oxides of d0 metals, VV, NbV, TaV, MoVI, and WVI, the olation process affords
polyoxometallate In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are ...
s, a large class of molecular metal oxides.


Oxygen-atom transfer

Metal oxo complexes are intermediates in many metal-catalyzed oxidation reactions. Oxygen-atom transfer is common reaction of particular interest in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
and
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
. Some metal-oxos are capable of transferring their oxo ligand to organic substrates. One such example of this type of reactivity is from and enzyme superfamily Molybdenum oxotransferase. In water oxidation catalysis, metal oxo complexes are intermediates in the conversion of water to O2.


Hydrogen-atom abstraction

Transition metal-oxo's are also capable of abstracting strong C–H, N–H, and O–H bonds.
Cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...
contains a high-valent iron-oxo which is capable of abstracting hydrogen atoms from strong C–H bonds.


Molecular oxides

Some of the longest known and most widely used oxo compounds are oxidizing agents such as
potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution. Potassium permanganate is widely used in the c ...
(KMnO4) and
osmium tetroxide Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the ...
(OsO4). Compounds such as these are widely used for converting alkenes to vicinal
diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...
s and alcohols to ketones or carboxylic acids. More selective or gentler oxidizing reagents include
pyridinium chlorochromate Pyridinium chlorochromate (PCC) is a yellow-orange salt (chemistry), salt with the chemical formula, formula 5H5NH rO3Clˆ’. It is a reagent in organic synthesis used primarily for organic redox reaction, oxidation of Alcohol (chemistry), al ...
(PCC) and
pyridinium dichromate The pyridinium dichromate (PDC) or Cornforth reagent is a pyridinium salt of dichromate with the chemical formula 5H5NHsub>2 r2O7 This compound is named after the Australian-British chemist Sir John Warcup Cornforth (b. 1917) who introduced it ...
(PDC). Metal oxo species are capable of catalytic, including asymmetric oxidations of various types. Some metal-oxo complexes promote C-H bond activation, converting hydrocarbons to alcohols. :


Metalloenzymes


Iron(IV)-oxo species

Iron(IV)-oxo compounds are intermediates in many biological oxidations: *
Alpha-ketoglutarate-dependent hydroxylases Alpha-ketoglutarate-dependent hydroxylases are a major class of non-heme iron proteins that catalyse a wide range of reactions. These reactions include hydroxylation reactions, demethylations, ring expansions, ring closures, and desaturations. Func ...
activate O2 by oxidative decarboxylation of
ketoglutarate Ketoglutaric acid or oxoglutaric acid, or its conjugate base, the carboxylate ketoglutarate or oxoglutarate, may refer to the following chemical compounds: * Ξ±-Ketoglutaric acid, an intermediate in the citric acid cycle The citric acid cycle (CA ...
, generating Fe(IV)=O centers, i.e. ferryl, that hydroxylate a variety of hydrocarbon substrates. *
Cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...
enzymes, use a
heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consisti ...
cofactor, insert ferryl oxygen into saturated C–H bonds, epoxidize olefins, and oxidize aromatic groups. *
Methane monooxygenase Methane monooxygenase (MMO) is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes (). There are two forms of MMO: the well-studied soluble form (s ...
(MMO) oxidizes methane to methanol via oxygen atom transfer from an iron-oxo intermediate at its non-heme diiron center. Much effort is aimed at reproducing reactions with synthetic catalysts.


Molybdenum/tungsten oxo species

The oxo ligand (or analogous sulfido ligand) is nearly ubiquitous in molybdenum and tungsten chemistry, appearing in the ores containing these elements, throughout their synthetic chemistry, and also in their biological role (aside from nitrogenase). The biologically transported species and starting point for biosynthesis is generally accepted to be oxometallates MoO4βˆ’2 or WO4βˆ’2. All Mo/W enzymes, again except
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrog ...
, are bound to one or more
molybdopterin Molybdopterins are a class of cofactors found in most molybdenum-containing and all tungsten-containing enzymes. Synonyms for molybdopterin are: MPT and pyranopterin-dithiolate. The nomenclature for this biomolecule can be confusing: Molybdopter ...
prosthetic group. The Mo/W centers generally cycle between hexavalent (M(VI)) and tetravalent (M(IV)) states. Although there is some variation among these enzymes, members from all three families involve oxygen atom transfer between the Mo/W center and the substrate. Representative reactions from each of the three structural classes are: *
Sulfite oxidase Sulfite oxidase () is an enzyme in the mitochondria of all eukaryotes, with exception of the yeasts. It oxidizes sulfite to sulfate and, via cytochrome c, transfers the electrons produced to the electron transport chain, allowing generation o ...
: SO3βˆ’2 + H2O β†’ SO4βˆ’2 + 2 H+ + 2 eβˆ’ *
DMSO reductase DMSO reductase is a molybdenum-containing enzyme that catalyzes reduction of dimethyl sulfoxide (DMSO) to dimethyl sulfide (DMS). This enzyme serves as the terminal reductase under anaerobic conditions in some bacteria, with DMSO being the ter ...
: H3CS(O)CH3 (
DMSO Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds an ...
) + 2 H+ + 2 eβˆ’ β†’ H3CSCH3 ( DMS) + H2O *
Aldehyde ferredoxin oxidoreductase In enzymology, an aldehyde ferredoxin oxidoreductase () is an enzyme that catalyzes the chemical reaction :an aldehyde + H2O + 2 oxidized ferredoxin an acid + 2 H+ + 2 reduced ferredoxin This enzyme belongs to the family of oxidoreductases, ...
: RCHO + H2O β†’ RCO2H + 2 H+ + 2 eβˆ’ The three different classes of molybdenum cofactors are shown in the Figure. The biological use of tungsten mirrors that of molybdenum.


Oxygen-evolving complex

The active site for the
oxygen-evolving complex The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the portion of photosystem II where photo-oxidation of water occurs during the light reactions of photosynthesis. The OEC is surrounded by four core protein subuni ...
(OEC) of
photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
(PSII) is a Mn4O5Ca centre with several bridging oxo ligands that participate in the oxidation of water to molecular oxygen. The OEC is proposed to utilize a terminal oxo intermediate as a part of the water oxidation reaction. This complex is responsible for the production of nearly all of earth's molecular oxygen. This key link in the
oxygen cycle Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) and the lithosphere (the Earth’s crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as wel ...
is necessary for much of the
biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
present on earth.


The "oxo wall"

The term "oxo wall" is a theory used to describe the fact that no terminal oxo complexes are known for metal centers with octahedral symmetry and d-electron counts beyond 5. Oxo compounds for the vanadium through iron triads (
Groups A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...
3-8) are well known, whereas terminal oxo compounds for metals in the cobalt through zinc triads (Groups 9-12) are rare and invariably feature metals with coordination numbers lower than 6. This trend holds for other metal-ligand multiple bonds. Claimed exceptions to this rule have been retracted. The iridium oxo complex Ir(O)(mesityl)3 may appear to be an exception to the oxo-wall, but it is not because the complex is non-octahedral. The trigonal symmetry reorders the metal d-orbitals below the degenerate MO Ο€* pair. In three-fold symmetric complexes, multiple MO bonding is allowed for as many as 7 d-electrons. Terminal oxo ligands are also rather rare for the titanium triad, especially zirconium and hafnium and are unknown for group 3 metals (scandium, yttrium, and lanthanum).


See also

* Metal-ligand multiple bond *
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 βˆ’2. Most of the E ...
*
Polyoxometalate In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are ...
*
Metallate Metallate or metalate is the name given to any complex anion containing a metal ligated to several atoms or small groups. Typically, the metal will be one of the transition elements and the ligand will be oxygen or another chalcogenide or a cyanid ...
*
Oxophilic Oxophilicity is the tendency of certain chemical compounds to form oxides by hydrolysis or abstraction of an oxygen atom from another molecule, often from organic compounds. The term is often used to describe metal centers, commonly the early trans ...
*
Dioxygen complex There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are: *A ...


References

{{Coordination complexes Ligands Transition metal oxides