Organonickel
Organonickel chemistry is a branch of organometallic chemistry that deals with organic compounds containing a nickel to carbon bond and their reactions  . They are used as a catalyst, as a building block in organic chemistry and in chemical vapor deposition. Organonickel compounds are also short-lived intermediates in organic reactions. The first organonickel compound ever discovered was nickel tetracarbonyl Ni(CO)4 in 1890 and quickly put to use in the Mond process for nickel purification.

# 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 60C  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)  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 compound 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 during which other catalysts that actually gave high mlar mass polymers (the actual Ziegler-Natta catalysts).
One practical implementation of alkyne oligomerization is the Reppe synthesis for example in the synthesis of cyclooctatetraene:
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 were formed quantitatively  :

## 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 160-200C with nickel(II) bromide and a copper halide.