The bis(μ-thiolato)dicobalt complex [Co2Cp2(μ-SMe)2] (1) reacts with alkyne-cobalt complexes [Co2(CO)6(μ-F3CCCR)] (2) to give tri- and tetranuclear cobalt cluster compounds. When R=CF3 the main product is [Co2(CO)4(μ-CF3C2CF 3)μ-Co2Cp2(μ-SMe)2] (3) but the trinuclear cluster [Co3Cp(CO)4(μ-SMe)2(μ-CF 3C2CF3)] (4) is also obtained in low yield. When R=H the products are the isomeric clusters [Co3Cp(CO)4(μ-SMe)2(μ-CF 3C2H)] (5 and 6), [Co3Cp2(CO)3(μ-SMe)(μ-CF 3C2CH)] (7) and [CpCo(CO)2]. The solid state structures of 3, 4, 5 and 7 have been established by X-ray analysis. The 50 electron triangular cluster 4 has weak Co-Co bonds [2.573(1), 2.800(1), 2.838(1) Å]. The alkyne ligand bridges the shortest of these bonds in perpendicular fashion. 5 and 7 contain open chain tricobalt units [Co-Co 2.39-2.57 Å] more typical of 50 electron M3 species. In these complexes the alkyne is π-bonded to the central metal atom and σ-bonded to both terminal cobalt atoms. Variable temperature NMR indicates that 5, 6 and 7 each exist in solution as interconverting isomers which differ in the configuration at one sulphur atom. For 5 the activation parameters [ΔH‡=64 kJ mol-1 and ΔS‡=-85 J K-1 mol-1] were obtained spectroscopically. In the solid, molecules of 7 contain an asymmetrically bridging carbonyl [Co-C 1.833(4) and 2.050(3) Å] which is not detectable in the solution spectra. © 2001 Elsevier Science B.V.