A colourless volatile liquid, Ni(CO)4; m.p. –25°C; b.p. 43°C. It is formed by direct combination of nickel metal with carbon monoxide at 50–60°C. The reaction is reversed at higher temperatures, and the reactions are the basis of the Mond process for purifying nickel. The nickel in the compound has an oxidation state of zero, and the compound is a typical example of a complex with pi-bonding ligands, in which filled d-orbitals on the nickel overlap with empty p-orbitals on the carbon.