Nickel carbonyl

Overview
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a colorless organometallic complex that is a versatile reagent, first described in 1890 by Ludwig Mond. It was the first metal simple carbonyl complex to be reported. Its volatility at room temperature and toxicity have earned the compound the nickname "liquid death."

Structure and bonding
Having the molecular formula Ni(CO)4, nickel carbonyl is composed of a central nickel atom surrounded by four carbonyl (carbon monoxide) ligands in a tetrahedral arrangement. 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 these compounds baffled chemists for many years, and most publications before 1950 depicted chains of CO chelated to the metal.

Nickel carbonyl has 18 valence electrons, like many other metal carbonyls such as iron pentacarbonyl and molybdenum hexacarbonyl. These metal carbonyls have symmetrical structures and are charge-neutral, resulting in their high volatility. In Ni(CO)4, the nickel atom has a formal oxidation number of zero.

Preparation
Ni(CO)4 was first synthesised in 1890 by Ludwig Mond by the direct reaction of nickel metal with CO. This pioneering work foreshadowed the existence of many other metal carbonyl compounds, including those of V, Cr, Mn, Fe, and Co.

Nickel metal reacts at room temperature with carbon monoxide gas to form the tetracarbonyl. At 323 K, carbon monoxide is passed over impure nickel. On moderate heating, such as contact with a hot glass surface, Ni(CO)4 decomposes back to carbon monoxide and nickel metal. These two reactions form the basis for the Mond process for the purification of nickel.

Chemical reactions
Like other low-valent metal carbonyls, Ni(CO)4 undergoes CO substitution reactions and can be oxidized. Donor ligands such as triphenylphosphine react to give Ni(CO)3(PPh3) and Ni(CO)2(PPh3)2. 2,2'-Bipyridine and related ligands behave similarly.

Chlorine oxidizes nickel carbonyl into NiCl2, releasing CO gas. Other halogens behave analogously. This reaction provides a convenient method for destroying unwanted portions of the toxic compound.

Reduction or treatment with hydroxides brings about clusters such as [Ni5(CO)12]2- and [Ni6(CO)12]2-.

Reactions of Ni(CO)4 with alkyl and aryl halides often result in cabonylated organic products. Vinyl halides, such as PhCH=CHBr, are converted to the unsaturated esters upon treatment with Ni(CO)4 followed by sodium methoxide. Such reactions also probably proceed via Ni(CO)3, which undergoes oxidative addition.

Metal carbonyls are also susceptible to attack by nucleophiles. Thus, treatment of Ni(CO)4 with some nucleophiles (Nu-) results in acyl derivatives including [Ni(CO)3C(O)Nu)]-.

Toxicology and safety considerations
Ni(CO)4 is highly hazardous, much more so than implied by its CO content, reflecting the effects of the nickel if it was released in the body. Nickel carbonyl may be fatal if absorbed through the skin or more likely, inhaled due to its high volatility. A folkloric account describes Ni(CO)4 as having a pleasant rose odor. Historically, laboratories that used Ni(CO)4 would keep a canary in the lab as an indicator of nickel carbonyl toxicity, due to the higher sensitivity of birds to this toxin. The vapours of Ni(CO)4 can autoignite.

Nickel carbonyl poisoning is characterized by a two-stage illness. The first consists of headaches and chest pain lasting a few hours, usually followed by a short remission. The second phase is a chemical pneumonitis which starts after typically 16 hours with symptoms of cough, breathlessness and extreme fatigue. These reach greatest severity after four days, possibly resulting in death from cardiorespiratory or renal failure. Convalescence is often extremely protracted, often complicated by exhaustion, depression and dyspnea on exertion. Permanent respiratory damage is unusual. The carcinogenicity of Ni(CO)4 is a matter of debate.