MMT
Other names: methylcyclopentadienyl manganese tricarbonyl, tricarbonyl(methyl-η5-cyclopentadienyl)manganese, CI-2, Combustion Improver-2
(C5H4Me)Mn(CO)3
45/50 
uncommon
Methylcyclopentadienyl manganese tricarbonyl (MMT) emerged as a replacement for the controversial additive tetraethyl lead. For much of the 20th century, lead was added to gasoline as an anti-knock agent, despite mounting evidence of lead's profound neurotoxicity and environmental persistence. As leaded gasoline was gradually phased out globally starting in the 1970s, MMT was introduced as an alternative octane booster. This orange-yellow liquid is a "half-sandwich" complex featuring a manganese atom coordinated a methylcyclopentadienyl ring and three carbonyl groups in a so-called "pianostool" geometry (the carbonyl ligands are like legs for the stool). Though some lingering questions remain about its safety, especially at high uses, MMT is still used as a combustion improver in gasoline in many countries, including the US, China, Canada, and Australia. MMT was responsible for the catastrophic thermal runaway and explosion at T2 Laboratories in Jacksonville, Florida in December 2007 that killed 4 people and injured 14.