Iron pentacarbonyl, Fe(CO)₅

Iron pentacarbonyl, Fe(CO)₅, was obtained by Mond and Langer by allowing finely divided iron, obtained by reduction of ferrous oxalate in a current of hydrogen, to remain twenty-four hours in an atmosphere of carbon monoxide. The gas was gradually absorbed, and, on warming the whole to 120° C., the carbonyl distilled over and condensed in a tube kept at -20° C.

As obtained in this way iron pentacarbonyl is a somewhat viscous liquid, pale yellow in colour, which solidifies at about -21° C. to a mass of yellow needle-shaped crystals. On heating the vapour it dissociates into metallic iron and carbon monoxide, dissociation being practically complete at 216° C.

The empirical formula, Fe(CO)₅, has been confirmed in several analyses, and the fact that the molecule is single and correctly represented by the above formula, when the carbonyl is dissolved in benzene, has been proved by cryoscopic measurements. The solid carbonyl melts at -19.5° to -20° C., and the liquid boils at 102.5° C., under 760 mm. pressure. Its density at various temperatures has been determined as follows: -

Temperature. °C.	Density.
0	1.4937
21.1	1.4565
40	1.4330
60	1.3825

Its mean coefficient of expansion with rise of temperature is 0.00138. Its vapour pressure at various temperatures is as follows: -

Temperature, °C.	-7	0	16.1	18.4	35	57	78
Vapour pressure in mm.	14.0	16.0	25.9	28.2	52.0	133.0	311.2

The halogens decompose solutions of the carbonyl in carbon tetrachloride with the formation of ferrous salts. In the presence of excess of chlorine, however, ferric chloride is formed. The velocity of reaction falls rapidly from chlorine to iodine.

Iron pentacarbonyl is adsorbed by metallic iron.

The fact that the halogens decompose the pentacarbonyl, yielding ferrous salts, apparently suggests that the iron is merely divalent in the carbonyl. If so, the structural formula is presumably: