Metal - Olefin complexes

Metal - Olefin complexes:-

Synopsis:- Definition,structure and bonding in alkene complexes,Different type of alkene complexes,catalysis,synthesis of alkene complexes ,reaction of alkene complexes,Rotational barrier,

Definition:-Alkene or olefin ligands are common in organotransition metal chemistry. In fact, the first organotransition metal complex, Zeise's salt (K[PtCl3(C2H4]·H2O) was an alkene complex .
While reacting K2PtCl4 with EtOH in 1827, the Danish chemist Zeise synthesized the famous Zeise’s salt K[PtCl3(C2H4)]•H2O containing a Pt bound ethylene moiety and which incidentally represented the first metal−olefin complex .

Structure and bonding in olefin complexes:-The metal−olefin bonding interaction is best explained by the Dewar−Chatt model, that takes into account two mutually opposing electron donation involving σ−donation of the olefinic C=C π−electrons to an empty dπ metal orbital followed by π−back donation from a filled metal dπ orbital into the unoccupied C=C π* orbital. Quite understandably so, for the d0 systems, the formations of metal−olefin complexes are not observed. The extent of the C=C forward π-donation to the metal and the subsequent π−back donation from the filled dπ orbital to the olefinic C=C π* orbital have a direct bearing on the C=C bond of the metal bound olefinic moiety in form of bringing about a change in hybridization as well as in the C−C bond distance .

The bonding in alkene complexes is described by the Dewar-Chatt-Duncanson model, which provides us with a bonding picture not unlike that seen in carbonyl or phosphine complexes. A sigma-type donation from the C=C pi orbital with concomitant pi-backbonding into an empty pi* orbital on the ethylene presents us with a synergistic bonding situation: the greater the sigma donation to the metal, the greater the pi-backbonding

Different type of alkene/olefin complexes:-

Dienes, trienes, polyenes, keto-alkenes, and other complicated alkene ligands

Butadiene, cyclooctadiene, and norbornadiene are well-studied chelating agents. Trienes and even some tetraenes can bind to metals through several adjacent carbon centers. Common examples of such ligands are cycloheptatriene and cyclooctatetraene. The bonding is often denoted using the hapticity formalism. Keto-alkenes are tetrahapto ligands that stabilize highly unsaturated low valent metals as found in (benzylideneacetone)iron tricarbonyl and tris(dibenzylideneacetone)dipalladium(0).

Metal alkene complexes. 

 Bis(cyclooctadiene)nickel(0), a catalyst and source of "naked nickel." 

The anion in Zeise's Salt, the first alkene complex. 

 Chlorobis(cyclooctene)rhodium dimer, source of "RhCl"Crabtree's catalyst, a very active catalyst for hydrogenation

Benzylideneacetone)iron tricarbonyl, source of "Fe(CO)3".

[Et3P]2Pt]6(η2-C60), a fullerene complex. 

 Mo(C7H8)(CO)3, a complex of cycloheptatriene. 

 Fe(C8H8)2, a complex of cyclooctatetraene.

Synthesis of Alkene Complexes:- 

Synthesis Metal alkene complexes are synthesized by the following methods.

 i. Substitution in low valent metals

ii.Reductin of high valent metal in th presence of an alkene

 iii. From alkyls and related species

Reaction of alkenes:- The metal alkene complexes show the following reactivities.

 i. Insertion reaction These reactions are commonly displayed by alkenes as they insert into metal−X bonds yielding metal alkyls. The reaction occurs readily at room temperature for X = H, whereas for other elements (X = other atoms), such insertions become rare. Also, the strained alkenes and alkynes

undergo such insertion readily.

  ii. Umpolung reactions Umpolung reactions are observed only for those metal−alkene complexes for which the metal center is a poor π−base and as a result of which the olefin undergoes a nuclophilic attack. iii. Oxidative addition Alkenes containing allyli .

Rotational barrier

The barrier for the rotation of the alkene about the M-centroid vector is a measure of the strength of the M-alkene pi-bond. Low symmetry complexes of ethylene, e.g. CpRh(C2H4)2, are suitable for analysis of the rotational barriers associated with the metal-ethylene bond. In Zeise's anion ([PtCl3(C2H4)]−) this rotational barrier cannot be assessed by NMR spectroscopy because all four protons are equivalentc hydrogens undergo oxidative addition to give a allyl hydride complex.

Catalysis

Metal alkene complexes are intermediates in many or most transition metal catalyzed reactions of alkenes: polymerization., hydrogenation, hydroformylation, and many other reactions. 

The mechanism of the Wacker process involves Pd-alkene complex intermediates.