Coupling of co-ordinated acetylide ligands with and without CO on chalcogen-stabilised mixed-metal clusters. Synthesis and characterisation of [M2Fe3(L)2(CO)6(μ3-E)2{μ-CC(Ph)C(Ph)C}] and [M2Fe2(L)2(CO)4(μ3-E)2{μ-CC(Ph)(CO)C(Ph)C}] (L = η5-C5Me5 or η5-C5H5, M = Mo or W, E = S, Se or Te)