Editing Carbon monoxide (section)

===Coordination chemistry===
{{Main|Metal carbonyl}}

[[File:MO COeng.svg|class=skin-invert-image|thumb|left|200px|Energy level scheme of the σ and π orbitals of carbon monoxide]]
[[Image:Carbon-monoxide-HOMO-phase-3D-balls.svg|thumb|right|150px|The [[HOMO/LUMO|HOMO]] of CO is a σ [[molecular orbital|MO]].]]
[[Image:Carbon-monoxide-LUMO-phase-3D-balls.png|thumb|right|150px|The [[HOMO/LUMO|LUMO]] of CO is a π* [[antibonding]] [[molecular orbital|MO]].]]

Most metals form [[coordination complex]]es containing covalently attached carbon monoxide. These derivatives, which are called [[metal carbonyls]], tend to be more robust when the metal is in lower oxidation states. For example [[iron pentacarbonyl]] ({{chem2|Fe(CO)5}}) is an air-stable, distillable liquid. [[Nickel carbonyl]] is a [[metal carbonyl complex]] that forms by the direct combination of carbon monoxide with the metal:<ref>{{cite book|title=Organometallics|author=C. Elschenbroich|publisher=VCH|year=2006|isbn=  978-3-527-29390-2}}</ref>
:{{chem2|Ni + 4 CO -> Ni(CO)4}} (1 [[Bar (unit)|bar]], 55 °C)
These volatile complexes are often highly toxic.  Some metal–CO complexes are prepared by decarbonylation of organic solvents, not from CO. For instance, [[iridium(III) chloride|iridium trichloride]] and [[triphenylphosphine]] react in boiling [[2-Methoxyethanol|2-methoxyethanol]] or [[dimethylformamide|DMF]] to afford {{chem2|IrCl(CO)(PPh3)2|link=Vaska's complex}}.

As a ligand, CO binds through carbon, forming a kind of triple bond. The lone pair on the carbon atom donates electron density to form a M-CO [[sigma bond]]. The two π* orbitals on CO bind to filled metal orbitals.  The effect is related to the [[Dewar-Chatt-Duncanson model]]. The effects of the quasi-triple M-C bond is reflected in the [[infrared spectrum]] of these complexes. Whereas free CO vibrates at 2143&nbsp;cm<sup>−1</sup>, its complexes tend to absorb near 1950&nbsp;cm<sup>−1</sup>.

[[Image:IronPentacarbonylStructure.png|class=skin-invert-image|150px|Structure of iron pentacarbonyl.]]