Metal Carbonyl Formation and Decomposition

While the equation for metal carbonyls is generally written as follows, the mechanism is far more complex than the equation implies:

At room temperature, carbonyls typically form liquids, vapors, and colored crystals. Carbonyls may be formed, volatilized, and condensed in a continuous process to separate them from the inert constituents of the feed. Some must be separated with elaborate techniques, while still others are intractable materials. They may be formed as liquids or as solutions in organic solvents and separated from inert solids by filtration. Crude carbonyls are separated from each other and residual impurities by distillation, sublimation, recrystallization, or selective solution of the carbonyl. Purified carbonyls are decomposed by heating into the carbon monoxide and pure metal. When a metal carbonyl can be separated from its impurities, subsequent decomposition generally results in high-purity metals. Exceptions to this include the formation of carbides.

Nickel tetracarbonyl was the first carbonyl discovered in the 1890s. Iron pentacarbonyl and cobalt octacarbonyl followed soon after; these remain the only carbonyls whose singular chemical properties have been applied commercially to extractive metallurgy, although it is speculated that ruthenium separation by carbonyl formation should ensue. In the formation of nickel tetracarbonyl, a mixture of freshly prepared nickel metal and nickel sulfide is heated in the presence of carbon monoxide.

Thermal decomposition reactions of commercial interest for cobalt, iron, and nickel carbonyls occur at temperatures about 200 °C (390 °F), where carbonyls can be handled in vapor form. Under these conditions, reaction kinetics permit acceptable powder production rates to be maintained. Carbonyls are heated rapidly to the desired decomposition temperature; at this temperature, nuclei form in the vapor to provide the required sites for metal deposition.

Decomposition products are the pure metal and carbon monoxide. During decomposition in commercial decomposers, the disproportionation reaction of carbon monoxide is catalyzed by the freshly formed metals. Iron is much more active as a catalyst than either nickel or cobalt, and therefore carbon contents in nickel carbonyl-derived powder are related to the trace iron content:

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