Osmium pentacarbonyl is the organoosmium compound with the formula Os(CO)5.

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.

[1] Samples of Os(CO)5 convert back to the trioosmium cluster upon heating to 80 °C.

The analogous conversion of Ru(CO)5 back to Ru3(CO)12 occurs at room temperature.