Bis(triethylphosphine)(η 4 -tetraphenylcyclobutadiene)nickel (4) was synthesized by the reduction of (η 4 -tetraphenylcyclobutadiene)nickel(II)bromide (3) with t-butyllithium in the presence of Et 3 P, and its structure was determined by X-ray crystallography. Furthermore, its reactivity towards CO, CH 3 CO 2 H, PhC≡CPh, LiAlH 4 and O 2 were investigated. 1,1-Bis(triethylphos-phine)-2,3,4,5-tetraphenylnickelole (14) was synthesized from (E,E)-1,4-dilithio-1,2,3,4-tetraphenyl-1,3-butadiene (15) and bis(triethylphosphine)nickel(II)bromide. Since the resulting crystals of the nickelole were not suitable for X-ray structure determination, the compound was characterized by elemental analyses, spectral data and carbonylation to yield tetraphenylcyclo-pentadienone (6). Analogous reductions of (η 4 -tetraphenylcyclobutadiene)nickel(II)bromide (3) in the presence of Ph 3 P or Ph 2 PCH 2 CH 2 PPh 2 , followed by carbonylation, led to 6 in 40% yield, demonstrating that about half of the cyclobutadiene rings in 3 undergo cleavage upon reduction to give the nickelole. Reactions of the dilithium reagent 15 with NiBr 2 complexed with Me 2 PCH 2 CH 2 PMe 2 ,Ph 3 P or Et 2 PCH 2 CH 2 PEt 2 , led to the formation of thermolabile nickeloles, as demonstrated by carbonylàtion which yielded 6. Warming of the nickeloles and subsequent treatment with CH 3 CO 2 H led to the formation of 1,2,3,4,5,6,7,8-octaphenyl-1,3,5,7-octatetraene (8) and, in one case, octaphenyl-cyclooctatetraene (5). The relevance of these findings to the mechanism of the Reppe nickel-catalyzed oligomerization of alkynes is discussed.