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| '''Dioxygen complexes''' are [[coordination compound]]s that contain O<sub>2</sub> as a [[ligand]].<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> The study of these compounds is inspired by oxygen-carrying proteins such as [[myoglobin]], [[hemoglobin]], [[hemerythrin]], and [[hemocyanin]].<ref>S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3.</ref> Several [[transition metal]]s form complexes with O<sub>2</sub>, and many of these complexes form reversibly.<ref>Berry, R. E. "Reactivity and Structure of Complexes of Small Molecules: Dioxygen", Comprehensive Coordination Chemistry II, 2004, 1, 625-629.</ref> The binding of O<sub>2</sub> is the first step in many important phenomena, such as [[cellular respiration]], [[corrosion]], and industrial chemistry. The first synthetic oxygen complex was demonstrated in 1938 with cobalt(II) complex reversibly bound O<sub>2</sub>.<ref>{{cite journal | author = Tokuichi Tsumaki | title = Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der [[Salen ligand|Oxyaldimine]] | journal = [[Bulletin of the Chemical Society of Japan]] | year = 1938 | volume = 13 | pages = 252–260 | doi = 10.1246/bcsj.13.252}}</ref>
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| ==Mononuclear complexes of O<sub>2</sub>== | |
| O<sub>2</sub> binds to a single metal center either “end-on” ([[hapticity|η<sup>1</sup>-]]) or “side-on” (η<sup>2</sup>-). The bonding and structures of these compounds are usually evaluated by single-crystal [[X-ray crystallography]], focusing both on the overall geometry as well as the O---O distances, which reveals its [[bond order]].
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| :[[Image:TMdioxygenCmpx.png|220px|center]]
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| ===Complexes of η<sup>1</sup>-O<sub>2</sub> ligands===
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| O<sub>2</sub> [[adduct]]s derived from [[cobalt]](II) and [[iron]](II) [[porphyrin]] complexes and related anionic ligands exhibit this bonding mode. Myoglobin and hemoglobin are famous examples, and many synthetic analogues have been described that behave similarly. Binding of O<sub>2</sub> is usually described as proceeding via [[electron transfer]] from the metal(II) center to give [[superoxide]] (O<sub>2</sub><sup>-</sup>) complexes of metal(III) centers.
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| ===Complexes of η<sup>2</sup>-O<sub>2</sub> ligands===
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| η<sup>2</sup>- bonding is the most common motif seen in coordination chemistry of dioxygen. Such complexes can generated by treating low-valent metal complexes with gaseous oxygen. For example, [[Vaska's complex]] reversibly binds O<sub>2</sub> (Ph = C<sub>6</sub>H<sub>5</sub>):
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| :IrCl(CO)(PPh<sub>3</sub>)<sub>2</sub> + O<sub>2</sub> <math>\overrightarrow{\leftarrow}</math> IrCl(CO)(PPh<sub>3</sub>)<sub>2</sub>O<sub>2</sub>
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| The conversion is described as a 2 e<sup>-</sup> [[redox]] process: Ir(I) converts to Ir(III) as dioxygen converts to peroxide. Since O<sub>2</sub> has a triplet ground state and Vaska's complex is a singlet, the reaction is slower than when [[singlet oxygen]] is used.<ref>Selke, M. and Foote, C. S., "Reactions of Organometallic Complexes with Singlet Oxygen. Photooxidation of Vaska's Complex", J. Am. Chem. Soc., 1993, volume 115, pages 1166 - 1167.{{DOI|10.1021/ja00056a061}}</ref>
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| Complexes containing η<sup>2</sup>-O<sub>2</sub> ligands are fairly common, but most are generated using [[hydrogen peroxide]], not O<sub>2</sub>. [[Chromate]] ([CrO<sub>4</sub>)]<sup>2-</sup>) can for example be converted to the tetraperoxide [Cr(O<sub>2</sub>)<sub>4</sub>]<sup>2-</sup>. The reaction of hydrogen peroxide with aqueous titanium(IV) gives a brightly colored peroxy complex that is a useful test for titanium as well as hydrogen peroxide.<ref>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.</ref>
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| ==Binuclear complexes of O<sub>2</sub>==
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| O<sub>2</sub> can bind to one metal of a bimetallic unit via the same modes discussed above for mononuclear complexes. A well-known example in nature is hemerythrin, which features a diiron carboxylate that binds O<sub>2</sub> at one Fe center. Dinuclear complexes can also cooperate in the binding, although the initial attack of O<sub>2</sub> probably occurs at a single metal. These binding modes include μ<sub>2</sub>-η<sup>2</sup>,η<sup>2</sup>-, μ<sub>2</sub>-η<sup>1</sup>,η<sup>1</sup>-, and μ<sub>2</sub>-η<sup>1</sup>,η<sup>2</sup>-. Depending on the degree of electron-transfer from the dimetal unit, these O<sub>2</sub> ligands can again be described as peroxo or superoxo. In nature, such dinuclear dioxygen complexes often feature copper.<ref>Lewis, E. A. and Tolman, W. B., "Reactivity of Dioxygen-Copper Systems", Chemical Reviews 2004, 104, 1047-1076. {{DOI|10.1021/cr020633r}}.</ref>
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| :[[Image:DimetalDioxgenComplexes.png|420px|center]]
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| ==Relationship to other oxygenic ligands and applications==
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| Dioxygen complexes are the precursors to other families of oxygenic ligands. Metal oxo compounds arise from the cleavage of the O-O bond after complexation. [[Hydroperoxide|Hydroperoxo]] complexes are generated in the course of the reduction of dioxygen by metals. The reduction of O<sub>2</sub> by metal catalysts is a key [[half-reaction]] in [[fuel cell]]s.
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| Metal-catalyzed oxidations with O<sub>2</sub> proceed via the intermediacy of dioxygen complexes, although the actual oxidants are often oxo derivatives. The reversible binding of O<sub>2</sub> to metal complexes has been used as a means to purify oxygen from air, but cryogenic distillation of [[liquid air]] remains the dominant technology.
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| ==References==
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| <references />
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| [[Category:Biochemistry]]
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| [[Category:Inorganic chemistry]]
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