Osmium pentacarbonyl is the organoosmium compound with the formula Os(CO)<sub>5</sub>. It is the simplest isolatable carbonyl complex of osmium. Osmium pentacarbonyl is a colorless volatile liquid that is obtained by treating solid triosmium dodecacarbonyl under 200 atmospheres of carbon monoxide at 280-290 ðC. In contrast, also at 200 atm of CO, solid Ru<sub>3</sub>(CO)<sub>12</sub> converts to Ru(CO)<sub>5</sub> at milder temperature of 160 ðC.
Samples of Os(CO)<sub>5</sub> convert back to the trioosmium cluster upon heating to 80 ðC. The analogous conversion of Ru(CO)<sub>5</sub> back to Ru<sub>3</sub>(CO)<sub>12</sub> occurs at room temperature. Chlorination of the pentacarbonyl gives a cationic pentacarbonyl complex:
Upon UV irradiation, hexane solutions of the pentacarbonyl react with ethylene to give mono-, di-, and trisubstituted derivatives: