Metal Carbonyls

compounds of metals with carbon monoxide, with the general formula Me_m (CO_n). Nickel car-bonyl, Ni(CO)₄, was the first to be discovered (1890), and since then the carbonyls of several metals and nonmetals have been prepared. Metal carbonyls may be either “mononuclear” or “polynuclear,” depending on the number of metal atoms in the molecule; compound metal carbonyls, such as [Co(CO)₄]₂Zn, also exist.

The carbonyls of nickel, iron, osmium, and ruthenium are liquids; most other carbonyls are crystalline. Metal carbonyls are diamagnetic, highly volatile, and extremely toxic. Only halide carbonyls, Me(CO)X, which are stable only in a carbon monoxide atmosphere, exist for copper, silver, and gold. Metal carbonyls decompose when heated above a certain temperature, releasing carbon monoxide and metal in a finely dispersed state. The physical properties of the principal metal carbonyls are presented in Table 1 (the carbonyls shown are readily soluble in organic solvents).

A common method of preparing metal carbonyls involves the reaction of carbon monoxide with metals or their salts at high temperatures and pressures. The metal carbonyls that are of the greatest commercial significance are nickel carbonyl, Ni(CO)₄; cobalt carbonyl, Co(CO)₄; and iron carbonyl, Fe(CO)₅. Carbonyls are used for the preparation of pure metals, which form upon their thermal dissociation. Thermal dissociation of cobalt, nickel, and chromium carbonyls is used in the application of metallic coatings, particularly on surfaces of complex shape. Cobalt and nickel carbonyls are catalysts in important chemical processes—for example, in the synthesis of carboxylic acids and their derivatives from olefins, and in the synthesis of acrylic acid from acetylene during hydroformylation:

Metal carbonyls are good antiknock compounds for motor fuels; however, the formation of oxides that are difficult to remove takes place during combustion. Certain carbonyls serve in the preparation of absolutely pure carbon monoxide.