The reaction of the labilized cluster [Os₃(CO)₁₀(MeCN)₂] and bis(4,6-dimethylpyrimidin-2-yl) disulfide affords two major products [Os₃(CO)₁₀(µ-dmpymt)₂]1(36%) and [Os₃(CO)₁₀(µ-SH)(µ-dmpymt)]2(19%)(dmpymt = 4,6-dimethylpyrimidine-2-thione). 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, [Os₃(CO)₁₀(µ-dmpymt)₂]3(62%), which contains two dmpymt ligands co-ordinated across the same non-bonding Os ⋯ Os edge. Thermolysis of cluster 3 in n-heptane for 2 h affords the cluster [Os₃(CO)₉(µ-dmpymt)(µ₃-η²-dmpymt)]4 as the major product (42%) in addition to a minor product [Os₃(CO)₈(µ-η²-dmpymt)(µ₃-η²-dmpymt)]5(9%). Cluster 4 contains two dmpymt moieties co-ordinating in µ and µ₃-η² modes respectively. Cluster 5 has two dmpymt moieties bonding in the µ₃-η² 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 Me₃NO in CH₂Cl₂ 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 [Os₃(CO)₉(µ-dmpymt)(µ-η²-dmpymt)]6 in almost quantitative yield (87%). Cluster 6 contains one dmpymt ligand co-ordinating in the µ-η² five-electron donating mode. Cluster 5 undergoes carbonylation to form cluster 4(62%) under a CO atmosphere in CH₂Cl₂ 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.