CARBONYLS, METALLIC, are compounds of the metals with carbon monoxide. Of these the most important is nickel carbonyl, used in the manufacture of pure nickel. This substance is readily produced by passing carbon monoxide over finely divided nickel at 3o-5o° C, and is condensed in a freezing mixture of ice and salt to a colourless liquid, boiling at C, and solidifying in acicular crystals at —25° C (L. Mond, 5890).
Prior to 1890 it had been observed in Mond's laboratory that, when exposed to an atmosphere contaminated with producer gas, nickel-plated taps and fittings became tarnished. This tarnish was ultimately traced to the action on the metal of the carbon monoxide contained in the producer gas. This observation led to systematic experiments on the interaction of carbon monoxide and nickel under varying conditions and thence to the discovery of nickel carbonyl, a volatile liquid having totally unsuspected prop erties and representing a new class of metallic derivatives. Mond and his co-workers then extended their researches to the allies of nickel and thus discovered the carbonyls of cobalt, iron, molyb denum and ruthenium. When passed through a hot tube at 18o-200° C, this carbonyl is dissociated with deposition of nickel as a brilliant mirror. Hence the use of the carbonyl in the extrac tion and purification of nickel, since the associated metals do not so readily furnish volatile carbonyls.
Under the conditions employed in the preparation of nickel carbonyl, cobalt yields no analogous compound; but above 15o° C and under a pressure of 3o-4o atmospheres, this metal unites with carbon monoxide to form cobalt tetracarbonyl an orange crystalline substance with the doubled molecular formula, which slowly dissociates at 6o° C into cobalt tricarbonyl, thus obtained it consists of sparingly soluble black crystals of un known molecular weight. Iron pentacarbonyl, a pale yellow viscid liquid, is produced by warming finely divided iron at too-12o° C in a slow current of carbon monoxide; it distils undecomposed at 102.8° C, and solidifies to a mass of yellow needles below —21° C. At 180° C it is completely dissociated. When exposed to sunlight below 6o° C, the pentacarbonyl becomes transformed into orange-red hexagonal plates of iron enneacar bonyl which are almost insoluble in ether, benzene or light petroleum. When these crystals are heated to between 50° and 6o° C in the presence of ether, ligroin or toluene, the liquid be comes green and deposits green prismatic crystals of iron tetra carbonyl , This compound, the analogue of nickel car bonyl, is stable under atmospheric conditions but at 14o-15o° C it dissociates into iron and carbon monoxide. Ruthenium carbonyl, Ru (CO) an orange-yellow deposit, is formed by passing car bon monoxide over ruthenium black. Molybdenum carbonyl, is obtained in highly refracting white crystals by the interaction of finely divided molybdenum and carbon monoxide at 200° C and under 200-250 atmospheres. These crystals sublime at 30-40° C in an atmosphere of hydrogen and carbon monoxide. Chromium is stated to yield a volatile carbonyl when carbon mon oxide is absorbed by magnesium phenyl bromide in presence of chromic chloride (A. Job and A. Cassal, 1927; See GRIGNARD REAGENTS) .
In addition to the foregoing compounds, where the metal is entirely associated with carbon monoxide, there are many complex metallic salts containing the carbonyl group, such as the carbonyl derivative of cuprous chloride, produced when the cuprous salt is employed in gas analysis. Platinous chloride furnishes carbonyl derivatives, e.g., which decomposes at 300° C into platinum and phosgene = Pt+COC12. Palladium forms a similar compound, The crude ferro cyanides from coal gas contain salts of carbonylferrocyanic acid, (G. T. M.)