Os 3 (CO) 1 0 (MeCN) 2 reacts with 4,6-dimethylpyrimidine-2-thione (dmpymt) in CH 2 Cl 2 at room temperature to give Os 3 (CO) 1 0 (μ-H)(μ-dmpymt) (1) and Os 3 (CO) 1 0 (μ-H)(μ-SH) (2). The corresponding reaction of Ru 3 (CO) 1 0 (MeCN) 2 with dmpymt gives Ru 3 (CO) 9 (μ-H)(μ 3 -η 2 -dmpymt) (3), Ru 3 (CO) 1 0 (μ-H)(μ-dmpymt) (4) and Ru 3 (CO) 1 0 (μ-dmpymt) 2 (5). Cluster 4 is thermally unstable and converts to 3 readily on standing. Clusters 1, 2, 3 and 5 are fully characterized by IR, 1 H NMR, MS and X-ray crystallography. Crystals of 1 are orthorhombic, space group Pnma with unit-cell dimensions a = 14.932(2),b = 16.290(2),c = 18.649(2) . Crystals of 2 are triclinic, space group P1, Z = 4, with unit-cell dimensionsa = 9.163(1), b = 14.123(3), c = 14.305(2) , α = 70.31(1), β = 89.32(1), γ = 76.60(1)°. Complex 3 crystallizes in the monoclinic space groupP2 1 c with a = 7.793(1),b = 15.558(1), c = 17.259(1) , β = 93.79(1)°. Complex 5 is monoclinic, space groupC2c , with a = 17.972(7), b = 12.988(6),c = 13.516(4) , β = 111.57(3)°.