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The reaction of Ni(PPh3)4with acetic and trifluoroacetic acids is studied by ESR. When Ni(PPh3)4reacts with protonic acids, oxidative addition of the acid to the Ni(0) occurs along with contradisproportionation of the formed Ni(II) complexes with the Ni(0) complex. As a result, stable Ni(I) carboxylate complexes (PPh3)3Ni–OOCR are formed, where R = CH3, CF3. The amounts of the formed Ni(I) carboxylate complexes and of the evolved hydrogen were found to depend on the rate of acid addition to the Ni(0) complex.