Nickel carbonyl, or tetracarbonylnickel is a  covalent compound of nickel, which is, unusually for a metal compound, a colorless liquid at room temperature. Its formula is Ni(CO)₄. It is highly toxic and may be fatal if ingested or absorbed through the skin, earning it the nickname "liquid death". Nickel carbonyl is extremely volatile. It evaporates at room temperature. The vapours can autoignite.  The volatile metalloid compound is used to refine nickel by the Mond process; also used to deposit nickel films and as a catalyst to produce organic chemicals. Nickel carbonyl has been identified in a variety of anthropogenic gases, e.g. landfill gas and sewage sludge digester gas and emissions of automobiles.