The dicobalt alkyne complex [Co2(μ-PhC≢CH)(CO)6] 1a reacted with [Mo2(η5-C5H5)2CO)4] in refluxing tetrahydrofuran (THF) to give the μ3-alkylidyne trinuclear cluster [Co2Mo(η5-C5H5)(μ3 -CCH2Ph)(CO)8] 3 as the major product. Possible routes to the formation of 3 have been investigated. Among the identified minor products from this reaction were [CoMo2(η5-C5H5)2(μ 3-CCH2Ph)(CO)7] 4, [Co2Mo2(η5-C5H5)2(μ4-PhC2H)(μ-CO)4(CO)4] 5 and [Co3Mo(η5-C5H5)2(μ 4-PhC2H)(μ-CO)2(CO)5] 6. In contrast, when the related dicobalt complexes [Co2(μ-CF3C≢CR) (CO)6] (R=CF3 (1b), H (1c)), which contain alkynes with more strongly electron-withdrawing substituents, were heated with [Mo2(η5-C5H5)2(CO)4] in THF only mixed-dinuclear complexes [(η5-C5H5)(CO)2Mo(μ-CF3C2R)Co(CO)3](Mo-Co) (R=H (7a), CF3 (7b)) were formed. The reaction of the mixed-dinuclear complex 7b with [Co(η5-C5H5)(CO)2] led to the mixed-tetranuclear butterfly cluster [Co2Mo2(η5-C5H5)3(μ-CO)(CO)3] 8. The thermal reaction of 1b with the phosphido compound [Mo2(η5-C5H5)2(μ-C O)(μ-PPh2)2] gave rise to a single product, [Mo2(η5-C5H5)2(μ-P Ph2)_μ-PPh2Co2(μ-CF3C2CF3)(CO)5₍CO)2] 9, in which a phenyl ring of the (μ-phosphido)dimolybdenum moiety is π-bonded to one of the two cobalt atoms. Spectroscopic data for the new complexes are discussed.