Organonickel

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Overview

Organonickel compounds can have oxidation state 0 or +2 and in many ways resemble organopalladium compounds in the same Group 10 element column of the periodic table. Both metals are reducing agents: Raney nickel for nickel and a host of palladium catalysts such as palladium on carbon. Many reactions of nickel compounds are based on reductive elimination / oxidative addition sequences.

Ni alkene complexes

Many complexes exist of nickel coordinated to an alkene. In these comounds nickel is formally zerovalent Ni⁰ and the bonding is based on the Dewar-Chatt-Duncanson model. One widely available representative is bis(1,5-cyclooctadiene)nickel(0) or Ni(COD)₂ containing two cyclooctadiene units. It is a 18VE compound with 10 electrons provided by nickel itself and 4x2 electrons more by the double bonds. This solid melts at 60°C ^[3] and is used as a catalyst.

Ni allyl complexes

Allyl halides react with Ni(CO)₄ to form nickel pi-allyl complexes. These compounds in turn are sources of allyl nucleophiles in reaction with alkyl halides. In allyl(cyclopentadienyl)nickel(II) ^[4] nickel has oxidation number +2. The electron count is 18 (8 from nickel, 6 from the Cp unit and 4 more from the allyl anion).

Nickelocenes

The nickel metallocene pendant is nickelocene NiCp₂ with +2 Ni oxidation state and with 20 valence electrons. It is easily reduced to a 19 VE nickelicinium ion. In contrast, the corresponding palladocene based on palladium is an unknown compound.

Nickel carbene complexes

With electron-rich alkenes nickel forms nickel transition metal carbene complexes containing C=Ni double bonds.

Nickel 12 VE compounds

Nickel compounds of the type NiR₂ also exist with just 12 valence electrons. In solution however solvent molecules always interact with the metal atom increasing the electron count. One true 12 VE compound is di(mesityl)nickel prepared from allyl nickel bromide and the corresponding Grignard reagent.

Organonickel Reactions

Alkene/alkyne oligomerizations

Nickel compounds catalyze the oligomerization of alkenes and alkynes. This property came to light as part as the development of Ziegler-Natta catalyst in the 1950s. It was found that nickel impurities originating from an autoclave killed the propagation reaction (Aufbau) in favor of termination reaction to a terminal alkene: the polymerization of ethylene simply stopped at 1-butene. This so-called nickel effect prompted the search for other catalyst capable of this reaction which resulted in the discovery of new catalysts that actually gave high molar mass polymers (the actual Ziegler-Natta catalysts).

One practical implementation of alkyne oligomerization is the Reppe synthesis for example in the synthesis of cyclooctatetraene:

This is a formal [2+2+2+2]cycloaddition. The oligomerization of butadiene with ethylene to trans-1,4-hexadiene at one time was an industrial process.

The formation of organonickel compounds in this type of reaction is not always obvious but in a carefully designed experiment two such intermediates are formed quantitatively ^{[5] [6]}:

It is noted in one study ^[7] that this reaction only works with acetylene itself or with simple alkynes due to poor regioselectivity. From a terminal alkyne 7 isomers are possibly differing in the position of the substituents or the double bond positions. One strategy to remedy this problem employs certain diynes:

The selected reaction conditions also minimize the amount formed of competing [2+2+2]]cycloaddition product to the corresponding substituted arene.

Coupling reactions

Nickel compounds cause the coupling reaction between allyl and aryl halides. Other coupling reactions involving nickel in catalytic amounts are the Kumada coupling and the Negishi coupling.

Ni carbonylation

Ni catalyzes the addition of carbon monoxide to alkenes and alkynes. The industrial production of acrylic acid at one time consisted of combining acetylene, carbon monoxide and water at 40-55 atm and 160-200°C with nickel(II) bromide and a copper halide.

See also

• Compounds of carbon with other elements in the periodic table:

References