Nickel is the actual
catalyst when small amounts of a nickel salt are added to the reaction. Nickel(II) chloride is first reduced to nickel(0) with 2 equivalents of
chromium(II) chloride (as
sacrificial catalyst) leaving
chromium(III) chloride. The next step is
oxidative addition of nickel into the carbon to halide bond forming an alkenylnickel R–Ni(II)–X intermediate followed by a
transmetallation step exchanging NiX with a Cr(III) group to an
alkenylchromium R–Cr(III)–X intermediate and regenerating Ni(II). This species reacts with the
carbonyl group in a
nucleophilic addition. : The amount of nickel used should be low as a direct alkene coupling to a
diene is a side reaction. Related reactions are the
Grignard reaction (magnesium), the
Barbier reaction (zinc) and addition reactions involving
organolithium reagents. ==References==