The triosmium carbonyl cluster, Os3(CO)10(µ2-OEt)2, exhibits unique chemistry when reacted with ethylene glycol which possesses two hydroxyl groups. Initially, a sole product is formed which has been identified as Os3(CO)8(µ2-OCH2CH2OH)2 where the glycol chains act as bidentate ligands. Formation of this product is entropically driven, stable only at higher temperatures. At room temperature the more enthalpically favored product Os3(CO)10(µ2-OCH2CH2OH)2 is formed. To obtain this structure, carbonyls are scavenged from other clusters creating carbonyl deficient minor products as well. These two major products exist in a reversible equilibrium. The addition of excess carbon monoxide to a solution of the eight carbonyl cluster will quickly push the equilibrium towards the enthalpically favored product, Os3(CO)10(µ2-OCH2CH2OH)2. Removal of the carbon monoxide atmosphere and an increase in the temperature will then shift the equilibrium back to the eight carbonyl cluster. It has also been shown that the exchange of the bridging alkoxy groups in Os3(CO)10(µ2-OEt)2 by ethylene glycol or isopropanol does not occur in the presence of carbon monoxide. A new mechanism for the substitution of the bridging groups has been proposed based on this observation. The Os3(CO)8(µ2-OCH2CH2OH)2 bidentate structure will react with PPh3 and PMe2Ph to form Os3(CO)8(µ2-OCH2CH2OH)2(PMe2Ph)2, where the glycol chain now only acts as a monodentate ligand. The enthalpic product is stable at room temperatures, but can be activated towards carbonyl substitution through the reversible process to form the bidentate structure. To our knowledge this is the first example of a cluster activated through an intramolecular weakly coordinated alcohol group.