Iron tetracarbonyl, Fe(CO)₄

When crystals of the nonacarbonyl are warmed with certain solvents to from 50° to 90° C., in an atmosphere of carbon dioxide, they dissolve, yielding green solutions which contain a third carbonyl, namely: -

Iron tetracarbonyl, Fe(CO)₄, the analogue of nickel carbonyl. Dewar and Jones obtained it in the form of dark green prismatic crystals from the green toluene solution. Iron tetracarbonyl is stable under ordinary conditions, but dissociates into metallic iron and carbon monoxide at 140° to 150° C. The crystals have a density of 1.996 at 18° C., and are soluble in toluene, light petroleum, etc., yielding dark green solutions which slowly decolorise at 100° C., and rapidly at 140° C., metallic iron being deposited. From cryoscopic measurements it appears that the tetracarbonyl has a high molecular weight when dissolved in benzene.