The bimetallic cobalt–molybdenum complex [(OC)₄CoMo(CO)₃(η⁵-C₅H₅)] reacted with phenylacetylene in benzene to give the bimetallic alkyne-bridged complex [(OC)₃Co(µ-PhCCH)Mo(CO)₂(η⁵-C₅H₅)]1 as the major product and the trimetallic alkylidyne-capped complex [Co₂Mo(µ₃-CCH₂Ph)(CO)₈(η⁵-C₅H₅)]2 as the minor product. Reaction of 1 with PPh₂(SPh) resulted in substitution of a carbonyl group on the cobalt atom to give [{(PhS)Ph₂P}(OC)₂Co(µ-PhCCH)Mo(CO)₂(η⁵-C₅H₅)]3. Thermolysis of 3 in refluxing toluene afforded, as the major product, the complex [η⁵-C₅H₅)(OC)Mo(µ-PPh₂CHCPh)(µ-SPh)Co(CO)₂]4, in which the unsymmetrical alkyne in 3 has been incorporated into a four- membered metallacycle, [graphic omitted]Ph, such that the phenyl substituent is positioned on the carbon atom remote from the phosphide group. The related complex [(η⁵-C₅H₅)(OC)Mo(µ-PPh₂CHCPh)(µ-SPh)Co(CO)(PPh₃)]5, in which one of the carbonyl groups on the cobalt atom in 4 has been substituted by a triphenylphosphine ligand, has also been isolated as a minor product from this reaction. Conversion of 4 into 5 has been achieved separately by reaction of 4 with PPh₃. Similarly, reaction of PPh₂(SPh) with 4 yields a related compound [(η⁵-C₅H₅)(OC)Mo(µ-PPh₂CHCPh)(µ-SPh)Co(CO){PPh₂(SPh)}]6, in which the same carbonyl group on the cobalt atom has been substituted by PPh₂(SPh). Single-crystal X-ray diffraction studies have been performed on complexes 2,4 and 6.