Cobalt Tetra-carbonyl, Co(CO)₄

Cobalt Tetra-carbonyl, Co(CO)₄ or Co₂(CO)₈. - This compound may be prepared by passing carbon monoxide at a pressure of 40 atmospheres over reduced cobalt at 150° C. The higher the pressure the more rapid is the formation of the carbonyl. It forms as fine, orange-coloured transparent crystals, which are best preserved by hermetically sealing them in a glass tube in an atmosphere of hydrogen or of carbon monoxide. Upon exposure to air decomposition takes place resulting in the formation of a basic cobalt carbonate. With bromine it yields cobalt bromide and carbon monoxide:

Co(CO)₄ + Br₂ = CoBr₂ + 4CO,

which suggests that the cobalt is divalent, and that the carbonyl is represented by the structural formula:



It is insoluble in water, and but slowly attacked by hydrochloric or sulphuric acid. Owing to its low vapour pressure and ready dis- sociability cobalt carbonyl is less liable to produce acute poisoning than the corresponding iron and nickel derivatives. The density of the crystals is 1.73, and their melting-point 51° C. The molecular weight determined cryoscopically in benzene solution, indicates that under these conditions the molecule is double, namely, Co₂(CO)₈. When heated at 60° C. one-quarter of the carbon monoxide is evolved, and jet-black crystals of Cobalt Tri-carbonyl, Co(CO)₃, are obtained. These may be purified by recrystallisation from benzene. Owing to their slight solubility their molecular weight could not be determined. At higher temperatures the tri-carbonyl decomposes to cobalt and carbon monoxide.