Transition metal oxo complex
In coordination chemistry, an oxo ligand is an oxygen atom bound only to one or more metal centers. These ligands can exist as terminal or (most commonly) as bridging atom (Fig. 1). Oxo ligands stabilize high oxidation states of a metal.
Oxo ligands are pervasive, comprising the great majority of the Earth's crust. This article concerns a subset of oxides, molecular derivatives. They are also found in several metalloenzymes, e.g. in the molybdenum cofactor and in many iron-containing enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is sodium ferrate (Na2FeO4) circa 1702.
Olation and acid-base reactions
Common reactions affected by metal-oxo compounds is olation, the condensation process that converts low molecular weight oxides to polymeric materials, including minerals. Olation often begins with the deprotonation of a metal-hydroxo complex.
Oxygen-atom transfer is another common reaction of particular interest in organic chemistry, but metal-oxides are capable of a variety of reactions including catalytic process. Examples here demonstrate the range of reactions of molecular metal oxo species. The formation of O2 by the oxygen evolving center can be viewed as an O-atom transfer from a manganese oxo group with water.
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, e.g. for converting alkenes to vicinal diols and alcohols to ketones or carboxylic acids. More selective or gentler oxidizing reagents include pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC), which have become widely used since the 1970s. Metal oxo species have been found capable of catalytic, including asymmetric oxidations of various types. Some metal-oxo complexes can activate C-H bonds to give an aldehyde or alcohol. Some metal oxides have been used to catalyze reduction of organic compounds.
Iron(IV)-oxo compounds are intermediates in many oxidations catalysed by heme-containing enzymes. 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. 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 solvent. Much of the effort in producing synthetic C-H bond activation catalysts has been inspired by these well designed natural 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 use. 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 molybdopterin 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 the Figure. The biological use of tungsten mirrors that of molybdenum.
The active site for the oxygen-evolving complex (OEC) of photosystem II (PSII) is a Mn4OxCa 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 is necessary for much of the biodiversity present on earth.
The "oxo wall"
The term "oxo wall" is theory used to describe the fact that no molecular oxo complexes is known for octahedral metal centers with d-electron counts beyond 4. This guideline recognizes that oxo groups are strong pi-donor ligands and require empty d-orbitals of pi-symmetry. A corollary is that the reactivity of complexes containing a terminal oxo ligand depends on the metal d-electron count. For octahedral d4 oxo complexes, the oxo ligand is nucleophilic. When the d-electron count is lower than 4, the oxo ligands become electrophilic. 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) 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.
Terminal oxo ligands are also rather rare for the titanium triad, especially zirconium and hafnium and is unknown for group 3 metals (scandium, yttrium, and lanthanum).
At first glance, the iridium oxo complex Ir(O)(mesityl)3 may appear to be an exception to the oxo-wall, but in this case, iridium is tetrahedral  and is d4 so there is no contradiction
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