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The oxo ligand is an oxygen atom bound only to a metal center (terminal) or centers (bridging) (Fig. 1). In synthetic inorganic and organometallic chemistry, the terminal oxo ligand is often used to stabilize high oxidation states of a given metal Oxo ligands occur naturally in minerals such as scheelite (calcium tungstate, CaWO4) and ferberite (iron(II) tunstate, FeWO4), the predominant minerals from which tungsten is purified. They are also found throughout biology in the molybdenum cofactor and in many important iron-oxo intermediates such as those found in the catalytic cycle for Cytochrome P450 enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is sodium ferrate (Na2FeO4) circa 1702.

Reactivity
The most common reactions carried out by metal-oxo compounds are oxidations or reductions via oxygen atom transfer, but metal-oxos are capable of quite a variety of reactivity and catalysis. The examples cited here are not meant to be comprehensive, but to demonstrate the types of reactions that can be done with metal oxo species.

Synthetic Compounds
Some of the longest known and most widely used oxo compounds are oxidizing agents such as potassium permanganate (KMnO4) and osmium tetroxide (OsO4). Compounds such as these have been known since the 1700s and are widely used in organic synthesis even to this day for oxidizing alkenes to vicinal diols or alcohols to ketones or carboxylic acids. More selective or gentler oxidizing reagents such as pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC) have also become widely used since the late 1970s. Metal oxo species have been found capable of catalytic and asymmetric oxidations of various types. Of particular interest in recent years are metal-oxo complexes which can activate C-H bonds to an aldehyde or alcohol. Metal-oxos such as Re(O)2I(PPh3)2 and Mo(O)2Cl2 have been used to catalytically reduce organic compounds. Reductions such as imines or amides to amines, carbonyls to alcohols, esters to alcohols or ethers, and even alkenes or alkynes via hydrogenation can all be catalyzed by metal-oxos.

Iron(IV)-Oxo
Iron(IV)-oxo is used as the active oxidizing species in iron-catalyzed oxidations throughout biology. One of the most widely studied examples is cytochrome p450 enzymes, which use a heme cofactor and commonly oxidize an alkyl group to an alcohol, a very difficult oxidation to do synthetically. Similarly, methane monooxygenase (MMO) oxidizes methane to methanol via oxygen atom transfer from an iron-oxo intermediate at its non-heme di-iron center. Oxygen atom transfer reactions are commonly done by metal-oxo species, but the enzyme systems mentioned have a few remarkable qualities that make them of particular interest. First, C-H bonds are quite resistant to oxidation and are generally unreactive at moderate temperatures (see C-H bond activation). Second, harsh oxidizing agents will generally oxidize an alcohol to a carboxylic acid, but these enzymes are able to oxidize an alkyl group to an alcohol without further oxidation to a carbonyl or carboxylic acid. The oxidant used in these enzymatic reactions is molecular oxygen in contrast with the harsh, toxic chemicals often found in conventional synthetic organic oxidations. As is generally the case with enzymatic reactions, these oxidations are chemically selective and take place at fast rates in aqueous sovent. Much of the effort in producing synthetic C-H bond activation catalysts has been inspired by these well designed natural catalysts.

Molybdenum/Tungsten
The oxo ligand (or analagous 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 use. In fact the biologically transported species and starting point for biosynthesis is generally accepted to be oxometallates MoO4-2 or WO4-2. All Mo/W enzymes except nitrogenase have the molybdenum cofactor prosthetic group which generally cycles between Mo(IV) and Mo(VI) in one electron steps. Though there is some variation among these enzymes, members from all three families involve oxygen atom transfer between the Mo center and the substrate. Representative reactions from each of the three structural classes are:
 * SO3-2 + H2O → SO4-2 + 2H+ + 2e- - (Sulfite oxidase)
 * H3CS(O)CH3 (DMSO) + 2H+ + 2e- → H3CSCH3 (DMS) + H2O - (DMSO reductase)

The three different classes of molybdenum cofactors are shown in Fig. 4. The biological use of tungsten mirrors that of molybdenum. Certain organisms, particularly hyperthermophiles, use tungsten in place of molybdenum to form the tungsten cofactor.

Oxygen Evolving Complex
The active site for the oxygen evolving complex (OEC) of Photosystem II (PSII) is a Mn4OxCa complex with several bridging oxo ligands that participate in the oxidation of water and release of molecular oxygen. The OEC is thought to go through 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 is necessary for much of the biodiversity present on earth.

The Oxo Wall
The term "oxo wall" was used to describe the fact that no metal oxo complexes for metals past the iron triad could be synthesized in spite of significant effort for over a decade. The stability of compounds containing a terminal oxo moiety varies greatly depending on the metal and its oxidation state. Oxo compounds for the vanadium through iron triads (groups 3-8) are well known, whereas terminal oxo compounds for metals in the cobalt through zinc triads (groups 9-12) were completely unknown (except for (mesityl)3Ir(O) - iridium oxo compound) until 2004. Late transition metals beyond the iron triad bind oxygen atoms almost exclusively in the bridging configuration. The same basic trend holds for other metal-ligand multiple bonds, though in some other instances is less strict. Terminal oxo is rather rare for the titanium triad, especially zirconium and hafnium and is unknown for group 3 metals (scandium, yttrium, and lanthanum), though the lack of metal oxos for the very early transition metals has received somewhat less attention than for the late transition metals.

Electronic Origin of the Oxo Wall
The simplest explanation for the presence of the oxo wall is that the terminal oxo ligand is stable for low d electron counts, but the stable oxidation states of late transition metals do not have low enough d counts to lead to stable oxo formation. The higher d counts of late transition metals lead to electrons in anti-bonding molecular orbitals and destabilization of the metal-oxygen bond (see Fig. 2). Generally, very high oxidation states are destabilized by electrostatics: a buildup of localized charge is always very destabilizing. Metals such as the group 6 metals (Cr, Mo, and W), which are very often found in the high +6 oxidation state, are stabilized by orbital considerations. The +6 oxidation state here yields a noble gas electron configuration, which is very stable from an electron orbital point of view, thus offsetting destabilizing electrostatic effects. The d orbitals for the very early transition are quite high energy and diffuse. Thus they do not match well in energy with the oxygen p orbitals, and in a metal-oxo interaction, the electron density tends to remain more localized on the oxygen in a more ionic type interaction. This makes the oxygen very nucleophilic and likely to attack other metal centers to yield a bridging configuration to delocalize the charge over multiple metals.

Exceptions
The first characterized iridium oxo compound was synthesized and characterized by Wilkinson et al. in 1993 This is a rare example of an Ir(V) d4 compound, thus it is isoelectronic with the more well known Fe(IV)-oxo examples. In some sense, it is most remarkable for its high oxidation state, which than naturally leads to stable oxo formation. Subsequent late transition metal-oxo compounds with platinum-, palladium- , and gold-oxo moieties were prepared in the research group of Craig Hill. These complexes contain polyoxometalate ligands which modulate the electronics of the late transition metal center and induce oxo formation via reaction with dioxygen. Polyoxometalates are essentially 3-dimensional oligomers of single metal center oxometallates, such as tungstate (WO4-2). They are generally electron poor, containing many oxygen atoms and metal centers often in a d0 configuration. As a result, they function as good π-acids, pulling electron density out of the metal orbitals that are involved in metal-oxo antibonding and stabilizing the complex. In addition, they have rather delocalized molecular orbitals that allow the extra electron density to be distributed over many atoms. The tuning of the electronics of the late transition metal center via appropriate polyoxometalate selection is rather delicate and resulting complexes are quite reactive electrophiles.