Methylcyclopentadienyl manganese tricarbonyl

Methylcyclopentadienyl manganese tricarbonyl (MMT) is an organometallic compound with the formula (CH3C5H4)Mn(CO)3. Marketed initially in 1958 as a supplement to the gasoline additive, tetraethyl lead to increase the fuel's octane rating, MMT was later used in unleaded gasoline. Although banned as a gasoline additive in the United States from 1977 to 1995, MMT has been used in Canadian gasoline since 1976 and was recently introduced in Australia. It is sold under the tradenames HiTec 3000 and AK-33X.

History
Tetraethyl lead (TEL) has been, and still is, used as an additive to increase the octane rating of gasoline. TEL was manufactured and marketed by the Ethyl Corporation, a joint venture between General Motors and DuPont. When the US Environmental Protection Agency (EPA) ordered the phase out of leaded gasoline in 1972, new fuel additives were sought. Initially marketed in 1958 as a smoke suppressant for gas turbines, MMT was used in 1974 as an additive in unleaded gasoline.

In 1977, use of MMT was banned in the US by the Clean Air Act until the Ethyl Corporation could prove that the additive would not lead to failure of new car emissions-control systems. As a result of this ruling, the Ethyl Corporation began a legal battle with the EPA, presenting evidence that MMT was harmless to automobile emissions-control systems. In 1995, the U.S. Court of Appeals ruled that the EPA had exceeded its authority and, as a result, MMT became a legal fuel additive in the US. MMT is nowadays manufactured by the Afton Chemical Corporation division of Newmarket Corporation.

Structure and bonding
The compound is called a half-sandwich compound. The manganese atom in MMT is bonded to three carbonyl groups as well as to the methylcyclopentadienyl ring. These hydrophobic organic ligands make MMT highly lipophilic, which may be relevant to bioacculumation. A variety of related complexes are known, including ferrocene, which is also under consideration as an additive to gasoline.

Safety
The health hazards associated with MMT have been hotly debated for decades. A 2003 study by the NICNAS in Australia suggested that MMT was highly toxic to humans in raw concentrated form, but ruled that the airborne concentrations of manganese as a result of car emissions from vehicles using fuel containing MMT posed no health hazard. A 2002 study by Masashi Kitazawa argued that dermal absorption from accidental gasoline spills, misuse of gasoline as a solvent cleaner, and deliberate gasoline fume inhalation were the main sources of potential MMT exposure.

Chronic exposure to manganese has been known to cause manganism. The symptoms of manganism are similar to those of Parkinson's disease. MMT has been shown to be cytotoxic and especially damaging to dopaminergic PC-12 cells. Toxicokinetic studies have shown that manganese derived from MMT absorbs into rat blood plasma at a rate 37 times that of inorganic manganese.

Immediate symptoms of MMT exposure are mild skin and eye irritation. Short-term symptoms resulting from MMT poisoning include giddiness, headache, neausea, and difficulties in breathing. Animal studies have shown that long-term exposure to MMT can result in damage to the liver and kidneys.