Some mechanisms of electrophilic substitution

Apart from the role of bonding with and stabilizing the cyclopentadienyl radicals, the iron atom may also take a direct part in the chemistry of ferrocene substituents, and in the mechanism of substitution. It has been proposed that electrophilic substitutions occur by a general reaction path in which the attacking electrophilic group interacts first with the iron atom to give a ir-diene cationic intermediate  

This mechanism explains why the relative reactivities in electrophilic substitution for different ferrocene derivatives parallels the ease of oxidation of the iron atom, both being dependent on the nature of the ring substituents. 

Similarly in electrophilic substitution of w-cyclobutadiene iron tricarbonyl, w-enyl (i.e. n-1 electron) intermediates have been postulated cf. p 195, e.g. 

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