We have studied the behaviour of transition metal carbonyl clusters in homogeneous and solid-gas hydrogenation catalysis trying to evidence structure-reactivity relationships. Here, we report on the behaviour of trinuclear homo- and hetero-metallic clusters containing "capping" sulphido or selenido ligands, as well as methylidyne ligands. Very modest results have been obtained in homogeneous hydrogenation reactions of diphenylacetylene. The low activity observed indicates that these complexes do not act as cluster catalysts. In effect of the capping ligands-which are supposed to stabilize the clusters and prevent fragmentation-do not act as expected. Formation and recondensation of metal fragments, to give inactive (and somewhat unexpected) reaction byproducts have been evidenced. These processes presumably occur at the expenses of fragment catalysis. Reaction pathways, supported by IR, H-1 NMR and mass spectrometric evidence are proposed. (C) 2003 Elsevier Science B.V. All rights reserved.