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Most palladium couplings start with a palladium salt with the palladium(II) oxidation state. The catalytic cycle requires palladium(0). The palladium salt is reduced in the reaction by the organometallic species. The organometallic can react with the catalyst, for example PdCl2, to form a diaryl palladium species, R-Pd-R. Palladium is still in the +2 oxidation state. From this state, the palladium can undergo a reductive elimination to form a new biaryl (R-R) and palladium(0). At this point, the palladium is in the proper oxidation state to participate in the catalytic cycle.
