The Negishi reaction involves catalytic addition of alkylzinc nucleophiles to vinyl halides.
Figure 1. The Negishi cross-coupling reaction.
The proposed catalytic cycle for this reaction involves the crucial oxidative addition of the vinyl halide to the metal. The alkylzinc probably delivers the alkyl nucleophile to the metal via conventional nucleophilic substitution. Once both species are on the metal, they experience reductive elimination together.
Figure 2. Catalytic cycle for the Negishi cross-coupling reaction.