Crystal structures have been determined of three derivatives of YCCo3(CO)9, all containing the tetrahedral CCo3 cluster of the parent compound: in PhCCo3(CO)6·π-C6H3Me3 a mesitylene molecule is π-bonded to a single Co atom of the Co3 triangle, having replaced three CO groups; in PhCCo3(CO)6·π-C8H8, a cyclo-octatetraene molecule has replaced the three axial CO groups of the parent molecule and is π-bonded to the Co3 triangle through three of its double bonds, and in the dimer, [CCo3(CO)9]2 the length of the central formally single C–C bond is 1.37(2)Å.