The co-ordinatively unsaturated dihydride cluster [Os-3(mu-H)(2)(CO)(10)] reacts with [Rh(COD)(L)I] (COD = cycloocta-1,5-diene; L=2,2'-bipyridine, 1,10-phenanthroline and 4,4'-diphenyl-2,2'-dipyridyl) to give a family of hydride heterometallic clusters [Os3Rh(mu-H)(3)(CO)(10)(eta(4)-COD)] 1, [Os3Rh(mu-H)(2)(CO)(8) (mu-CO)(mu-I)(eta(4)-COD)] 2, [Os3Rh(mu-H)(2)(CO)(8)(mu-CO)(mu-I)(bipy)] 3, [Os3Rh(mu-H)(2)(CO)(8)(mu-CO)(mu-I)(1,10-phen)] 4, [Os3Rh(mu-H)(2)(CO)(8)(mu-CO)(mu-I)(4,4'-diphbipy)] 5 and [Os-3(CO)(12)] in moderate yields. The crystal structures of 1-3 were established by X-ray diffraction method; 1 and 2 comprise a saturated tetrahedral Os3Rh core in which the rhodium atom is eta(4)-co-ordinated by a COD ligand. In structure 3-5, a bidentate N-donor ligand chelates to the rhodium metal centre instead of the COD ligand. Clusters 1-5 were fully characterized by IR and H-1-NMR spectroscopy, mass spectrometry and elemental analysis. (C) 1998 Elsevier Science S.A. All rights reserved.