The roles of nickel and chromium catalysts in the coupling reaction of vinyl halides and aldehydes, the so-called Nozaki–Hiyama–Kishi (NHK) reaction, have been studied by UV/Vis spectroscopy, electrochemical, and spectroelectrochemical methods. Electrochemical studies revealed that nickel plays the central role in activating the vinyl halide by reductive cleavage, to form a rapidly decomposing vinyl–Ni species. The latter can, however, be stabilized in the presence of the Cr complex. The redox behavior of the Ni complexes in the presence of vinyl halide demonstrated that the vinyl halide activation results from interaction with a one-electron reduced nickel species [formally NiI], not necessarily with a Ni0 species. It was furthermore shown by UV/Vis spectroscopy and spectroelectrochemical methods that low-valent nickel [Ni0] results from the interaction of the NiII catalyst with CrCl2.
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