Why not use other organometallic compounds?
As the toxicity of the alkyl lead and the halogenated scavengers became of concern, alternatives were considered. The most famous of these is methylcyclopentadienyl manganese tricarbonyl (MMT), which was used in the USA until banned by the EPA from 27 Oct 1978 [18], but is approved for use in Canada and Australia. It is more expensive than alkyl leads and has been reported to increase unburned hydrocarbon emissions and block exhaust catalysts [19]. Other compounds that enhance octane have been suggested, but usually have significant problems such as toxicity, cost, increased engine wear etc.. Examples include dicyclopentadienyl iron and nickel carbonyl.
