Osmium pentacarbonyl

Osmium pentacarbonyl is the organoosmium compound with the formula Os₅. 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 °C. In contrast, also at 200 atm of CO, solid Ru₃(CO)₁₂ converts to Ru(CO)₅ at milder temperature of 160 °C.

Reactions

Samples of Os₅ convert back to the trioosmium cluster upon heating to 80 °C. The analogous conversion of Ru₅ back to Ru₃₁₂ 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: