What is "methylcyclopentadienyl manganese tricarbonyl"

n. (context organic compound automotive English) (CH₃C₅H₄)Mn(CO)₃ ; an octane number boosting additive to unleaded gasoline to replace tetra-ethyl lead as an anti-knock component

Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (CHCH)Mn(CO). Initially marketed as a supplement for use in leaded gasoline, MMT was later used in unleaded gasoline to increase the octane rating. Following the implementation of the Clean Air Act (United States) (CAA) in 1970, MMT continued to be used alongside tetraethyl lead (TEL) in the US as leaded gasoline was phased out (prior to TEL finally being banned from US gasoline in 1995), and was also used in unleaded gasoline until 1977. Ethyl Corporation obtained a waiver from the U.S. EPA (Environmental Protection Agency) in 1995, which allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/L (manganese per liter).

MMT has been used in Canadian gasoline since 1976 (and in numerous other countries for many years) at a concentration up to 18 mg Mn/L (though the importation and interprovincial trade of gasoline containing MMT was restricted briefly during the period 1997-1998) and was introduced into Australia in 2000. It has been sold under the tradenames HiTEC® 3000, Cestoburn and Ecotane.