Abstract Studies have been made of photochemical and thermal reaction sequences through which bisubstituted acetylenes are transformed in (C 5H 5)Co-carbonyl reaction systems into cyclobutadiene and cyclopentadienone complexes and hexasubstituted benzenes. A primary intermediate observed by its IR spectrum in low-temperature photochemical reactions of (C 5H 5)Co(CO) 2 with diphenyl alkynes RCCR is the mixed mononuclear species (C 5H 5)Co(CO)(RCCR). At room temperature this species is converted by excess alkyne into the cyclopentadienone complex (C 5H 5)Co(R 4C 4CO). We have isolated from these reactions systems an important intermediate the mixed binuclear compound(C 5H 5) 2Co 2(μ-CO)(RCCR). In the presence of excess alkyne this compound is thermally converted either to the cyclobutadiene or to the cyclopentadienone complex of (C 5H 5)Co, depending on the partial pressure of CO in the reaction system. The mixed binuclear compound forms a catalyst for the cyclotrimerization of excess 2-butyne. The fluxional binuclear metallocycle (C 5H 5) 2Co 2[(CH 3) 4C 4], which is formed by sodium amalgam reduction of (C 5H 5)Co(CO)I 2 in the presence of 2-butyne, is a true catalyst for alkyne cyclotrimerization.