Methyl-, phenyl-, and fluoro-methinyltricobalt enneacarbonyls react directly, or in ether solvents, with arenes to give the labile π-arene complexes, YCCo3(CO)6(π-arene). The i.r., n.m.r., and mass spectra show that the solid-state structure of the arene co-ordinated symmetrically to one cobalt atom is retained in solution. Tetranuclear acetylene complexes YC⋮CYCo4(CO)10(Y = Me or Ph) are also formed in these reactions; it is suggested that the acetylene moiety is derived from the parent clusters.