Nonaromatic Alkene and Alkyne Complexes

Complexes between metal salts and alkenes have been known since 1827 but they were not understood until the latter half of this century. For example, Zeise isolated stable yellow crystals after refluxing an alcoholic solution of platinum tetrachloride.71 Zeise s salt is now made from K2PtCl4 and C2H4  

Silver ions form similar alkene complexes which are soluble in aqueous solution and may be used to effect the separation of unsaturated hydrocarbons from alkanes. Catalysts for the polymerization of alkenes also form metal-alkene complexes which lead to polymerized product. 

A structural investigation of the anion in Zeise s salt has shown that the ethylene occupies the fourth coordination site of the square planar complex wth the C—C axis perpendicular to the platinum-ligand plane (Fig. 15.22).75 Relative to free ethylene, the C—C bond is lengthened slightly (from 133.7 pm to 137.5 pm), and the hydrogens are slightly tilted back from a planar arrangement. 

In extreme cases, such as Pt(Ph3P)2(CH2==CH2) in which the metal is very electron rich, v back donation is significant and as a result, the carbon-carbon bond lengthens considerably (to 143 pm) and the hydrogen atoms are bent considerably oul of the plane  

In this molecule ethylene lies in the same plane as the other ligands, unlike the case in Zeise s salt where planarity is slerically prevented. By analogy to a three-membered ring of carbon atoms, this compound may be viewed as a metallacyclopropane. 

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Metal Alkyls, Carbenes, Carbynes, and Carbides 655 Nonaromatic Alkene and Alkyne Complexes 662 Metallocenes 669... 

A cascade Heck reaction with termination by nucleophiles is considered to start with an oxidative addition of a heteroatom-carbon bond (starter) onto a palladium(O) species (startup reaction), followed by carbopalladation of a nonaromatic carbon-carbon double or triple bond without subsequent dehydropalladation (relay), a second and possibly further carbopalladation of a carbon-carbon double or triple bond (second etc. relay). The terminating step is a displacement of the palladium residue by an appropriate nucleophile. It is crucial for a successful cascade carbopalladation that no premature dehydropalladation takes place, and that can be prevented by using alkynes and 1,1-disubstituted alkenes (or certain cycloalkenes) as relay stations since they give kinetically stable alkenyl- or neopentylpalladium intermediates, respectively. In addition, reaction of haloalkenes with alkenes in certain cases may form rr-allyl complexes, which are then trapped by various nucleophiles. 

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