The reaction of HCoRu3(CO)13 with H2 in refluxing hexane gives the new cluster H3CoRu3(CO)12 in 75% yield. Infrared and NMR spectra indicate that the cluster exists in two isomeric forms. The structure of one isomer has been determined by single-crystal X-ray diffraction. It crystallizes in the triclinic space group P1 with a = 9.204 (2) Å, b = 13.615 (4) Å, c = 8.318 (2) Å, a = 100.49 (2)°, β = 111.51 (2)°, γ = 81.78 (2)°, V = 950.1 Å3 and Z = 2. The structure was determined by the heavy-atom method and refined to R1 = 2.3% and R2 = 3.6%. The CoRu3 core of the molecule defines a distorted tetrahedron with three carbonyls terminally bound to each metal atom. The hydride ligands were located by difference Fourier techniques and bridge the three Ru-Ru bonds. The overall symmetry of the cluster is approximately C3v. Spectral evidence indicates that the second isomer contains a bridging CO ligand and three nonequivalent hydrogens. The interconversion of the two isomers and the various hydrogen-exchange processes have been monitored by 1H NMR spectroscopy, and the exchange mechanisms are discussed.