F. William Sunderman, Jr.

Occurrence and Uses

Metal carbonyls have the general formula Me_x(CO)_y, and are formed by combination of the metal (Me) with carbon monoxide (CO). Physical properties of some metal carbonyls are listed in table 1. Most are solids at ordinary temperatures, but nickel carbonyl, iron pentacarbonyl and ruthenium pentacarbonyl are liquids, and cobalt hydrocarbonyl is a gas. This article focuses on nickel carbonyl, which, because of its volatility, exceptional toxicity and industrial importance merits special attention in regard to occupational toxicology. Since iron pentacarbonyl and cobalt hydrocarbonyl also have high vapour pressures and potential for inadvertant formation, they warrant serious consideration as possible occupational toxicants. Most metal carbonyls react vigorously with oxygen and oxidizing substances, and some ignite spontaneously. Upon exposure to air and light, nickel carbonyl decomposes to carbon monoxide and particulate nickel metal, cobalt hydrocarbonyl decomposes to cobalt octacarbonyl and hydrogen, and iron pentacarbonyl decomposes to iron nonacarbonyl and carbon monoxide.

Table 1. Physical properties of some metal carbonyls

*Decomposition starts at temperature shown.

Source: Adapted from Brief et al. 1971.

Metal carbonyls are used in isolating certain metals (e.g., nickel) from complex ores, for producing carbon steel, and for metallizing by vapour deposition. They are also used as catalysts in organic reactions (e.g., cobalt hydrocarbonyl or nickel carbonyl in olefin oxidation; cobalt octacarbonyl for the synthesis of aldehydes; nickel carbonyl for the synthesis of acrylic esters). Iron pentacarbonyl is used as a catalyst for various organic reactions, and is decomposed to make finely powdered, ultra pure iron (so-called carbonyl iron), which is used in the computer and electronics industries. Methycyclopentadienyl manganese tricarbonyl (MMT) (CH₃C₅H₄Mn(CO)₃) is an antiknock additive to gasoline and is discussed in the article “Manganese”.

Health Hazards

The toxicity of a given metal carbonyl depends on the toxicity of carbon monoxide and of the metal from which it is derived, as well as the volatility and instability of the carbonyl itself. The principal route of exposure is inhalation, but skin absorption can occur with the liquid carbonyls. The relative acute toxicity (LD₅₀ for the rat) of nickel carbonyl, cobalt hydrocarbonyl and iron pentacarbonyl may be expressed by the ratio 1:0.52:0.33. Inhalation exposures of experimental animals to these substances induce acute interstitial pneumonitis, with pulmonary oedema and capillary damage, as well as injury to the brain, liver and kidneys.

Judging from the sparse literature on their toxicity, cobalt hydrocarbonyl and iron pentacarbonyl rarely pose health hazards in industry. None the less, iron pentacarbonyl can be formed inadvertently when carbon monoxide, or a gas mixture containing carbon monoxide, is stored under pressure in steel cylinders or fed through steel pipes, when illuminating gas is produced by petroleum reforming, or when gas welding is carried out. Presence of carbon monoxide in emission discharges from blast furnaces, electric arc furnaces and cupola furnaces during steel-making can also lead to the formation of iron pentacarbonyl.

Safety and Health Measures

Special precautions are mandatory in the storage of metal carbonyls; their handling must be mechanized to the maximum degree, and decanting should be avoided wherever possible. Vessels and piping should be purged with an inert gas (e.g., nitrogen, carbon dioxide) before being opened, and carbonyl residues should be burnt or neutralized with bromine water. Where there is an inhalation hazard, workers should be provided with airline respirators or self-contained breathing apparatus. Workshops should be fitted with down-draught ventilation.

Nickel Carbonyl

Nickel carbonyl (Ni(CO)₄) is mainly used as an intermediate in the Mond process for nickel refining, but it is also used for vapour-plating in the metallurgical and electronics industries and as a catalyst for synthesis of acrylic monomers in the plastics industry. Inadvertent formation of nickel carbonyl can occur in industrial processes that use nickel catalysts, such as coal gasification, petroleum refining and hydrogenation reactions, or during incineration of nickel-coated papers that are used for pressure-sensitive business forms.

Hazards

Acute, accidental exposure of workers to inhalation of nickel carbonyl usually produces mild, non-specific, immediate symptoms, including nausea, vertigo, headache, dyspnoea and chest pain. These initial symptoms usually disappear within a few hours. After 12 to 36 hours, and occasionally as long as 5 days after exposure, severe pulmonary symptoms develop, with cough, dyspnoea, tachycardia, cyanosis, profound weakness and often gastrointestinal symptoms. Human fatalities have occurred 4 to 13 days after exposure to nickel carbonyl; deaths have resulted from diffuse interstitial pneumonitis, cerebral hemorrhage or cerebral oedema. In addition to pathologic lesions in the lungs and brain, lesions have been found in liver, kidneys, adrenals and spleen. In patients who survive acute nickel carbonyl poisoning, pulmonary insufficiency often causes protracted convalescence. Nickel carbonyl is carcinogenic and teratogenic in rats; the European Union has classified nickel carbonyl as an animal teratogen. Processes that use nickel carbonyl constitute disaster hazards, since fire and explosion can occur when nickel carbonyl is exposed to air, heat, flames or oxidizers. Decomposition of nickel carbonyl is attended by additional toxic hazards from inhalation of its decomposition products, carbon monoxide and finely particulate nickel metal.

Chronic exposure of workers to inhalation of low atmospheric concentrations of nickel carbonyl (0.007 to 0.52 mg/m³) can cause neurological symptoms (e.g., insomnia, headache, dizziness, memory loss) and other manifestations (e.g., chest tightness, excessive sweating, alopecia). Electroencephalographic abnormalities and elevated serum monoamine oxidase activity have been observed in workers with chronic exposures to nickel carbonyl. A synergistic effect of cigarette smoking and nickel carbonyl exposure on the frequency of sister-chromatid exchanges was noted in a cytogenetic evaluation of workers with chronic exposure to nickel carbonyl.

Safety and Health Measures

Fire and explosion prevention. Because of its flammability and tendency to explode, nickel carbonyl should be stored in tightly closed containers in a cool, well-ventilated area, away from heat and oxidizers such as nitric acid and chlorine. Flames and sources of ignition should be prohibited wherever nickel carbonyl is handled, used or stored. Nickel carbonyl should be transported in steel cylinders. Foam, dry chemical, or CO₂ fire extinguishers should be used to extinguish burning nickel carbonyl, rather than a stream of water, which might scatter and spread the fire.

Health protection. In addition to the medical surveillance measures recommended for all nickel-exposed workers, persons with occupational exposures to nickel carbonyl should have biological monitoring of nickel concentration in urine specimens on a regular basis, typically monthly. Persons who enter confined spaces where they might possibly be exposed to nickel carbonyl should have self-contained breathing apparatus and a suitable harness with lifeline tended by another employee outside the space. Analytical instruments for continuous atmospheric monitoring of nickel carbonyl include (a) Fourier-transform infrared absorption spectroscopes, (b) plasma chromatographs and (c) chemiluminescent detectors. Atmospheric samples can also be analysed for nickel carbonyl by (d) gas chromatography, (e) atomic absorption spectrophotometry and (f) colourimetric procedures.

Treatment. Workers suspected to have been acutely exposed to nickel carbonyl should be immediately removed from the exposure site. Contaminated clothing should be removed. Oxygen should be administered and the patient kept at rest until seen by a physician. Each voiding of urine is saved for nickel analysis. The severity of acute nickel carbonyl poisoning correlates with the urine nickel concentrations during the first 3 days after exposure. Exposures are classified as “mild” if the initial 8-h specimen of urine has a nickel concentration less than 100 µg/l, “moderate” if the nickel concentration is 100 to 500 µg/l, and “severe” if the nickel concentration exceeds 500 µg/l. Sodium diethyldithiocarbamate is the drug of choice for chelation therapy of acute nickel carbonyl poisoning. Ancillary therapeutic measures include bed rest, oxygen therapy, corticosteroids and prophylactic antibiotics. Carbon monoxide poisoning may occur simultaneously and requires treatment.

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