Nickel carbonyl (IUPAC name: tetracarbonylnickel) is the organonickel compound with the chemical 
formula Ni(CO)4. This pale-yellow metal carbonyl is very volatile at room temperature and highly toxic. Nickel Carbonyl can be used to nickel coat steel and other metals and to make very pure nickel. It is an intermediate in the Mond process for the purification of nickel and is a reagent in organometallic chemistry.
Nickel carbonyl is a tetrahedral molecule with four carbonyl (carbon monoxide) ligands attached to nickel. The CO ligands, in which the C and the O are connected by triple bonds (often depicted as double bonds), are covalently bonded to the nickel atom via the carbon ends. The structures of nickel carbonyl and related compounds baffled chemists for many years,[why?] and most publications before 1950 depicted chains of CO chelated to the metal.