Transition metal alkyne complex
In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.[1]
Synthesis
Transition metal alkyne complexes are formed by the displacement of other ligands by the alkyne. For example, a variety of cobalt-alkyne complexes may be formed by reaction of the alkyne with dicobalt octacarbonyl.[2]
- Co2(CO)8 + R2C2 → Co2(C2R2)(CO)6 + 2CO
Structure and Bonding
The coordination of alkynes to transition metals is similar to that of alkenes. The bonding can be described by the Dewar-Chatt-Duncanson model.
η2-coordination to a single metal center
When bonded side-on to a single metal atom, an alkyne acts as a dihapto two-election donor. The complex can be described as a metallacyclopropene. π-backbonding also occurs, with a filled d-orbital on the metal donating electron density to the π* antibonding orbital of the alkyne. As a result of this interaction, metal-carbon bond lengths decrease, and the substituents of the alkyne fold away from the metal by 30 to 40°.[3][4] Illustrative is the complex (diphenylacetylene)bis(triphenylphosphine)platinum. In this complex, the Pt-C bond is 2.06 Å, the C-C bond of the alkyne is 1.32 Å, and the C-C-C bond angle of the alkyne and one of its phenyl substituents is 140°.[5]
η2, η2-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 η2-diphenylacetylene-(hexacarbonyl)dicobalt(0).[4]
References
- ^ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 3-527-29390-6
- ^ 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
- ^ Hill, A.F. Organotransition Metal Chemistry, 2002, Royal Society of Chemistry, ISBN 0-471-28163-8.
- ^ a b Crabtree, R.H. Comprehensive Organometallic Chemistry V, 2009, John Wiley & Sons, Inc. ISBN 0-470-25762-3.,
- ^ Glanville, J.O.; Stewart, J.M.; Grim, S.O.; "Structure of bis(triphenylphosphine)diphenylacetylene platinum" J. Organomet. Chem. 1967, 7, 9-10. doi:10.1016/S0022-328X(00)90844-1
