NiBr2 reacts with 3-hexyne in the presence of Mg and EtOH to give the cyclobutadiene bromide complex [(C4Et4)NiBr2]2, which serves as a general precursor for various cyclobutadiene nickel compounds. In particular, complexes with 2-electron ligands (C4Et4)Ni(L)Br2 (L = PPh3, P(OMe)3, pyridine), complexes with bidentate ligands [(C4Et4)Ni(L2)Br]PF6 (L2 = bipyridine, phenanthroline), and nickelacarborane (C4Et4)NiC2B9H11 were obtained from [(C4Et4)NiBr2]2 in 70-95% yields. The reactions of [(C4Et4)NiBr2]2 with an excess of strong ligands, such as tBuNC or dppe, led to the displacement of cyclobutadiene. The structures of the five compounds have been established by X-ray diffraction analysis. The previously reported data on cyclobutadiene nickel complexes have been also reviewed.