The heteronuclear tetrahedral cluster derivative (η-H)Os3W(CO)12(η5-C5H5) has been studied by means of a single-crystal X-ray structural analysis. The complex crystallizes in the noncentrosymmetric orthorhombic space group [FORMULA OMMITED] Diffraction data were collected with a Syntex P21 diffractometer and the structure was refined to RF=5.6% and RwF=4.6% for those 1446 reflections with [FORMULA OMMITED] [Mo Kαradiation]. The molecule contains a tetrahedral cluster of metal atoms in which the tungsten is coordinated to an η5-C5H5 ligand and two terminal carbonyl ligands and in which each osmium is associated with three terminal carbonyl ligands. There is, in addition, a bridging carbonyl ligand across the Os(1)-W(4) bond and a bridging hydride ligand across the Os(2)-Os(3) vector. Consideration of bond lengths and cis M-M-L angles reveal that the bridging hydride ligand lies in, or close to, the Os(1)-Os(2)-Os(3) plane. © 1979, American Chemical Society. All rights reserved.