Ferrocene derivatives conjugated systems

Reversible one-electron oxidation of ferrocene and its derivatives to cation radicals (so-called ferricenium cations) is a well-known reaction. The cation radical center is localized at the iron atom. In contrast to this statement, the hole transfers though conjugated systems were proven for the bis(ferrocenyl) ethylene cation radical (Delgado-Pena et al. 1983) and the cation radical of bis(fulvaleneiron) (LeVanda et al. 1976). Scheme 1-53 depicts these structures. 

Special procedures for visualising ferrocene derivatives are unnecessary since they are coloured. Alkylferrocenes and derivatives without chromophore groups in conjugation with the ferrocene nucleus are yellow monoacylferrocenes are orange diacyl derivatives, red and decidedly conjugated systems, red-violet. Less intensively coloured ferrocene derivatives may be oxidised to blue or blue-green ferrocinium derivatives. The lower limit of detection of most ferrocence derivatives is 2-3 [xg. 

The butadiene moiety in (IV) is presumed to be planar, or nearly so, and the iron atom lies below this plane and approximately equidistant from the four carbon atoms of the diene system. The nature of the diene-iron bonding was presumed to involve interaction of the Fe atomic orbitals with -n molecular orbitals of the diene system as a whole and was therefore more analogous to the 77 bonding in ferrocenes, rather than the a type implied in Reihlen s structure (II). The conjugated nature of the diene system was considered to be an essential feature necessary for the formation of iron derivatives of this type. 

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