Transition metal alkene complex
In organometallic chemistry, a transition metal alkene complex is a coordination compound containing one or more alkene ligands. Such compounds are intermediates in many catalytic reactions that convert alkenes to other organic products.
Mono- and dialkenes are often used as ligands in stable complexes.
The simplest monoalkene is ethylene. Many complexes of ethylene are known, including Zeise's salt (see figure), Rh2Cl2(C2H4)4, Cp*2Ti(C2H4), and the homoleptic Ni(C2H4)3. Substituted monoalkene include the cyclic cyclooctene, as found in chlorobis(cyclooctene)rhodium dimer. Alkenes with electron-withdrawing groups commonly bind strongly to low-valent metals. Examples of such ligands are TCNE, tetrafluoroethylene, maleic anhydride, esters of fumaric acid. These acceptors form adducts with many zero-valent metals.
Cyclooctadiene and norbornadiene are popular chelating agents. Keto-alkenes are polyhapto ligands that stabilize highly unsaturated low valent metals as found in (Benzylideneacetone)iron tricarbonyl and tris(dibenzylideneacetone)dipalladium(0).
Trienes and even some tetraenes can bind to metals through several adjacent carbon centers. Common examples of such ligands are cycloheptatriene and cyclooctatetrene. The bonding is often denoted using the hapticity formalism.
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