Organonickel chemistry is a branch of organometallic chemistry that deals with organic compounds featuring nickel-carbon bonds. Organonickel compounds are used as catalysts, as building blocks in organic chemistry, and in chemical vapor deposition. They are also short-lived intermediates in organic reactions. The first organonickel compound, nickel tetracarbonyl Ni(CO)4, was reported in 1890 and was quickly applied in the Mond process for nickel purification. Organonickel complexes are prominent in numerous industrial processes, including carbonylations, hydrocyanation, and the Shell Higher Olefin Process.

Classes of compounds

Alkyl and aryl complexes
A popular reagent is Ni(CH3)2(tetramethylethylenediamine) (NiMe2(TMEDA)). Many alkyl and aryl complexes are known with the formula NiR(X)L2. Examples include (dppf)Ni(cinnamyl)Cl, trans-(PCy2Ph)2Ni(o-tolyl)Cl, (dppf)Ni(o-tolyl)Cl, (TMEDA)Ni(o-tolyl)Cl, and (TMEDA)NiMe2. Nickel compounds of the type NiR2 also exist with just 12 valence electrons. In solution, however, solvents always interact with the metal atom, increasing the electron count. One 12-electron compound is di(mesityl)nickel, prepared from (allyl)2Ni2Br2 and the corresponding Grignard reagent:
(allyl)2Ni2Br2 + 4 C6H2Me3MgBr → 2 (allyl)MgBr + 2 MgBr2 + 2 (C6H2Me3)2Ni

Alkene complexes
Many complexes exist of nickel coordinated to an alkene. Practical applications of this theme include polymerization or oligomerization of alkenes, as in the Shell Higher Olefin Process. In these compounds nickel is formally zerovalent, Ni(0), and the bonding is described by the Dewar–Chatt–Duncanson model. A common representative is bis(cyclooctadiene)nickel(0) (Ni(COD)2), which contains two cyclooctadiene ligands. It is an 18-electron (18e-) compound, with 10 electrons provided by nickel and 4×2 electrons from the double bonds. This solid, which melts at 60 °C, is used as a catalyst and as a precursor for many other nickel compounds.

Allyl complexes
Nickel forms several simple allyl complexes. Allyl halides react with Ni(CO)4 to form π-allyl complexes such as (allyl)2Ni2Cl2; these compounds are sources of allyl nucleophiles. In (allyl)2Ni2Br2 and (allyl)Ni(C5H5), nickel is assigned oxidation number +2, and the electron counts are 16 and 18, respectively. Bis(allyl)nickel is prepared from allylmagnesium bromide and nickel chloride.

Cyclopentadienyl complexes
Nickelocene, NiCp2, contains Ni in the +2 oxidation state and is a 20-valence-electron metallocene. It can be oxidized by one electron. The corresponding palladocene and platinocene are unknown. From nickelocene many derivatives are generated, e.g. CpNiLCl, CpNiNO, and Cp2Ni2(CO)3.

Carbene complexes
Nickel forms carbene complexes that formally feature C=Ni double bonds.

Reactions — alkene/alkyne oligomerizations
Nickel compounds catalyze the oligomerization of alkenes and alkynes, a property that helped validate the research and development of Ziegler–Natta catalysts in the 1950s. The discovery that nickel impurities from an autoclave suppressed the propagation (Aufbau) step and promoted termination to produce a terminal alkene was striking: the polymerization of ethylene suddenly stopped at 1‑butene. This "nickel effect" prompted the search for other catalysts and ultimately led to systems capable of producing high‑molar‑mass polymers, such as modern Ziegler–Natta catalysts. One practical implementation of alkyne oligomerization is the Reppe synthesis, for example in the formation of cyclooctatetraene, a formal [2+2+2+2] cycloaddition. Oligomerization of butadiene with ethylene to give trans‑1,4‑hexadiene was once an industrial process. Formal [2+2+2] cycloadditions also occur in alkyne trimerization, which can be extended to include benzyne: benzyne is generated in situ from an ortho‑trimethylsilyl aryl triflate and reacts with a diyne such as 1,7‑octadiyne in the presence of a NiBr2·bis(diphenylphosphino)ethane/Zn catalyst system to give the corresponding naphthalene derivative. In the catalytic cycle, elemental zinc reduces Ni(II) to Ni(0), which can then coordinate two alkyne bonds. A cyclometalation step converts the nickel cyclopentadiene intermediate into a species that coordinates benzyne, which undergoes C–H insertion to give a nickel cycloheptatriene compound. Reductive elimination then liberates a tetrahydroanthracene product. The formation of organonickel intermediates in this type of reaction is not always obvious, but in a carefully designed experiment two such intermediates were formed quantitatively. One study noted that the reaction only works with acetylene or simple alkynes because more substituted alkynes give poor regioselectivity. From a terminal alkyne, seven isomers are possible, differing in the positions of substituents or double bonds. One strategy to remedy this problem is to employ certain diynes; the selected reaction conditions also minimize formation of the competing [2+2+2] cycloaddition product, which gives the corresponding substituted arene.

Nickel compounds promote coupling reactions between allyl and aryl halides. Other nickel-catalyzed coupling reactions include the Kumada and Negishi couplings.

Nickel catalyzes the addition of carbon monoxide to alkenes and alkynes. Industrially, acrylic acid was once produced by reacting acetylene, carbon monoxide, and water at 40–55 atm and 160–200 °C in the presence of nickel(II) bromide and a copper halide.

See also: nickel(IV) organometallic complexes; nickel(II) precatalysts; lactate racemase.