Abstract Bis(triethylphosphine)(η4-tetraphenylcyclobutadiene)nickel (4) was synthesized by the reduction of (η4-tetraphenylcyclobutadiene)nickel(II)bromide (3) with t-butyllithium in the presence of Et3P, and its structure was determined by X-ray crystallography. Furthermore, its reactivity towards CO, CH3CO2H, PhC≡CPh, LiAlH4 and O2 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 (η4 -tetraphenylcyclobutadiene)nickel(II)bromide (3) in the presence of Ph3P or Ph2PCH2CH2PPh2 , 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 NiBr2 complexed with Me2PCH2CH2PMe2 ,Ph3P or Et2PCH2CH2PEt2 , led to the formation of thermolabile nickeloles, as demonstrated by carbonylàtion which yielded 6. Warming of the nickeloles and subsequent treatment with CH3CO2H 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.