The reaction of the labilized cluster [Os,(CO),,( MeCN),] and bis(4.6-dimethylpyrimidin-2-yl) disulfide affords two major products [O~,(CO),~(p-drnpymt),] 1 (36%) and [O~~(CO),,(p-SH)(p-dmpymt)] 2). Cluster 2 is formed from C-S bond cleavage of one of the dmpymt ligands at ambient conditions. Thermolysis of cluster 1 in n-heptane (98 "C) for 1 h produces an isomeric cluster of 1, [Os,(CO),,(p-dmpymt),] 3 (62%). which contains two dmpymt ligands co-ordinated across the same non-bonding Os...Os edge. Thermolysis of cluster 3 in nheptane for 2 h affords the cluster [Os,(CO),(p-dmpymt) (p3-qz-dmpymt)] 4 as the major product (42%) in addition t o a minor product [Os,(CO),(p-q2-dmpymt)(p3-qz-dmpymt)] 5 (9%). Cluster 4contains two dmpymt moieties co-ordinating in p and p3-q2 modes respectively. Cluster 5 has two dmpymt moieties bonding in the p3-q2 and p-q2 modes respectively. Further thermolysis of cluster 4 in n-octane (125°C) leads to a much higher yield of cluster 5 (60%) at the expense of cluster 4. Chemical activation of cluster 1 using Me,NO in CH,CI, at -78°C also produces clusters 4 and 5 in 20 and 15% yield respectively. A new, orange product (31%) is also formed but remains uncharacterized. When cluster 4 is allowed to stand under ambient conditions over a period of ten days, it isomerizes to the cluster [Os,(CO),(p-dmpymt) (p-q2-dmpymt)] 6 in almost quantitative yield (87%). Cluster 6 contains one dmpymt ligand co-ordinating in the p-q2 five-electron donating mode.Cluster 5 undergoes carbonylation to form cluster 4 (62%) under a CO atmosphere in CH,CI, while cluster 4 also changes back to cluster 3 (47%) in refluxing n-hexane (69 "C) under a CO atmosphere. Formation of clusters 3-5 is thus reversible. Mechanisms for the formation of 3, 4 and 6 have also been proposed. All the clusters 1-6 isolated have been fully characterized by conventional spectroscopic methods as well as single-crystal X-ray analyses.