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In [[organometallic chemistry]], a '''transition metal alkyne complex''' is a [[coordination compound]] containing one or more alkyne [[ligand]]s.  Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. [[hydrogenation]] and [[alkyne trimerization|trimerization]].<ref>Elschenbroich, C. ”Organometallics” 2006 Wiley-VCH: Weinheim. {{ISBN|3-527-29390-6}}.</ref>

==Synthesis==
Transition metal alkyne complexes are often formed by the displacement of labile ligands by the alkyne.  For example, a variety of [[dicobalt hexacarbonyl acetylene complex|cobalt-alkyne complexes]] arise by the reaction of alkynes with dicobalt octacarbonyl.<ref>Kemmitt, R. D. W.; Russell, D. R.; "Cobalt" in ''Comprehensive Organometallic Chemistry I''; Abel, E.W.; Stone, F.G.A.; Wilkinson, G. eds., 1982, Pergamon Press, Oxford.  {{ISBN|0-08-025269-9}}</ref>
:{{chem2|Co2(CO)8  +  R2C2  ->  (R2C2)Co2(CO)6  +  2 CO}}

Many alkyne complexes are produced by reduction of metal halides:<ref>{{cite journal|doi=10.1021/om0208570|title=The Titanocene Complex of Bis(trimethylsilyl)acetylene: Synthesis, Structure, and Chemistry|year=2003|last1=Rosenthal|first1=Uwe|last2=Burlakov|first2=Vladimir V.|last3=Arndt|first3=Perdita|last4=Baumann|first4=Wolfgang|last5=Spannenberg|first5=Anke|journal=Organometallics|volume=22|issue=5|pages=884–900}}</ref>
: Cp<sub>2</sub>TiCl<sub>2</sub> + Mg + Me<sub>3</sub>SiC≡CSiMe<sub>3</sub> → Cp<sub>2</sub>Ti[(CSiMe<sub>3</sub>)<sub>2</sub>] + MgCl<sub>2</sub>

==Structure and bonding==
[[File:VarietyPackAlkyneCmpx.svg|thumb|center|620px|Structures of various metal-alkyne complexes.]]
The coordination of alkynes to transition metals is similar to that of alkenes. The bonding is described by the [[Dewar–Chatt–Duncanson model]]. Upon complexation the C-C bond elongates and the alkynyl carbon bends away from 180º. For example, in the phenylpropyne complex Pt(PPh<sub>3</sub>)<sub>2</sub>(MeC<sub>2</sub>Ph), the C-C distance is 1.277(25) vs 1.20 Å for a typical alkyne.  The C-C-C angle distorts 40° from linearity upon complexation.<ref>{{cite journal|first1=B. William |last1=Davies |last2=Payne|first2=N. C. |title=Studies on Metal-Acetylene Complexes: V. Crystal and Molecular Structure of Bis(triphenylphosphine)(1-phenylpropyne)platinum(0), [P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>]<sub>2</sub>(C<sub>6</sub>H<sub>5</sub>CCCH<sub>3</sub>)Pt<sup>0</sup>|journal=J. Organomet. Chem.|year=1975| volume=99|page=315|doi=10.1016/S0022-328X(00)88462-4}}</ref>  Because the bending induced by complexation, strained alkynes such as cycloheptyne and cyclooctyne are stabilized by complexation.<ref>{{cite journal|title=Metal Complexes of Small Cycloalkynes and Arynes |first1=Martin A. |last1=Bennett |first2=Heinz P. |last2=Schwemlein |journal=[[Angew. Chem. Int. Ed. Engl.]] |date=1989 |volume=28|issue=10 |pages=1296–1320|doi=10.1002/anie.198912961}}</ref>

The C≡C vibration of alkynes occurs near 2300&nbsp;cm<sup>−1</sup> in the IR spectrum.  This mode shifts upon complexation to around 1800&nbsp;cm<sup>−1</sup>, indicating a weakening of the C-C bond.

===η<sup>2</sup>-coordination to a single metal center===
When bonded side-on to a single metal atom, an alkyne serves as a dihapto usually two-electron donor.  For early metal complexes, e.g., Cp<sub>2</sub>Ti(C<sub>2</sub>R<sub>2</sub>), strong π-backbonding into one of the π* antibonding orbitals of the alkyne is indicated. This complex is described as a metallacyclopropene derivative of Ti(IV). For late transition metal complexes, e.g., Pt(PPh<sub>3</sub>)<sub>2</sub>(MeC<sub>2</sub>Ph), the π-backbonding is less prominent, and the complex is assigned oxidation state 0.<ref>Hill, A.F. ''Organotransition Metal Chemistry'', 2002, Royal Society of Chemistry, {{ISBN|0-471-28163-8}}.</ref><ref name=Crabtree>Crabtree, R. H. ''Comprehensive Organometallic Chemistry V'', 2009, John Wiley & Sons {{ISBN|978-0-470-25762-3}} {{Verify source|date=May 2015}}</ref>

In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal.  This kind of bonding was first implicated in complexes of the type W(CO)(R<sub>2</sub>C<sub>2</sub>)<sub>3</sub>.<ref>{{cite journal|author=Joseph L. Templeton|title=Four-Electron Alkyne Ligands in Molybdenum(II) and Tungsten(II) Complexes|journal=Advances in Organometallic Chemistry'|year=1989|volume=29|page=1–100|doi=10.1016/S0065-3055(08)60352-4}}</ref>

===η<sup>2</sup>, η<sup>2</sup>-coordination bridging two metal centers===
Because alkynes have two π bonds, alkynes can form stable complexes in which they bridge two metal centers.  The alkyne donates a total of four electrons, with two electrons donated to each of the metals.  And example of a complex with this bonding scheme is η<sup>2</sup>-diphenylacetylene-(hexacarbonyl)dicobalt(0).<ref name="Crabtree"/>

===Benzyne complexes===
[[Transition metal benzyne complex]]es represent a special case of alkyne complexes since the free benzynes are not stable in the absence of the metal.<ref>William M. Jones, Jerzy Klosin "Transition-Metal Complexes of Arynes, Strained Cyclic Alkynes, and Strained Cyclic Cumulenes" Advances in Organometallic Chemistry 1998, Volume 42, Pages 147–221. {{doi|10.1016/S0065-3055(08)60543-2}}</ref>

==Applications==
Metal alkyne complexes are intermediates in the semi[[hydrogenation]] of alkynes to alkenes:
:C<sub>2</sub>R<sub>2</sub> + H<sub>2</sub> → ''cis''-C<sub>2</sub>R<sub>2</sub>H<sub>2</sub>

This transformation is conducted on a large scale in refineries, which unintentionally produce acetylene during the production of ethylene. It is also useful in the preparation of fine chemicals. [[Semihydrogenation]] affords cis alkenes.<ref>{{cite journal |author1=Michaelides, I. N. |author2=Dixon, D. J.|title=Catalytic Stereoselective Semihydrogenation of Alkynes to E-Alkenes|journal=Angew. Chem. Int. Ed.|year=2013|volume=52|issue=3|pages=806–808|doi=10.1002/anie.201208120|pmid=23255528|doi-access=free}}</ref>

Metal-alkyne complexes are also intermediates in the metal-catalyzed [[alkyne trimerisation|trimerization]] and tetramerizations. [[Cyclooctatetraene]] is produced from acetylene via the intermediacy of metal alkyne complexes. Variants of this reaction are exploited for some syntheses of substituted [[pyridine]]s. 

The [[Pauson–Khand reaction]] provides a route to cyclopentenones via the intermediacy of cobalt-alkyne complexes.
[[File:Pauson Khand reaction original.svg|center|PK reaction]]

[[Acrylic acid]] was once prepared by the [[hydrocarboxylation]] of acetylene:<ref name=Ullmann>{{ Ullmann | author = W. Bertleff |author2=M. Roeper |author3=X. Sava | title = Carbonylation | doi = 10.1002/14356007.a05_217}}</ref>
:C<sub>2</sub>H<sub>2</sub> +  H<sub>2</sub>O + CO → H<sub>2</sub>C=CHCO<sub>2</sub>H
With the shift away from coal-based (acetylene) to petroleum-based feedstocks (olefins), catalytic reactions with alkynes are not widely practiced industrially. 

[[Polyacetylene]] has been produced using metal catalysis involving alkyne complexes.
:[[File:Ziegler natta scheme for polyacetylene.png|thumb|left|Ti-catalyzed polymerization of [[acetylene]], inspired by [[Ziegler–Natta catalysis]].]]

[[Cuprous chloride]] also catalyzes the [[Dimer (chemistry)|dimerization]] of [[acetylene]] to [[vinylacetylene]], once used as a precursor to various polymers such a [[neoprene]].  Mechanistic studies suggest that this reaction proceeds by insertion of acetylene into a copper(I) [[acetylide]] complex.<ref>{{cite journal |doi=10.1021/cr400357r |title=Catalytic Reactions of Acetylene: A Feedstock for the Chemical Industry Revisited |date=2014 |last1=Trotuş |first1=Ioan-Teodor |last2=Zimmermann |first2=Tobias |last3=Schüth |first3=Ferdi |journal=Chemical Reviews |volume=114 |issue=3 |pages=1761–1782 |pmid=24228942 |doi-access=free }}</ref>

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==References==
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{{Organometallics}}{{Coordination complexes}}
[[Category:Organometallic chemistry]]
[[Category:Transition metals]]
[[Category:Coordination chemistry]]