Ferrocene ability to stabilize a carbenium ion

The ability of ferrocene to stabilize positive charge has been recognized by several authors. Ferrocenylmethylium ions have been isolated as stable salts, e.g., as tetrafluoroborates. Several research groups have studied the kinetics of their reactions with different nucleophiles. Crystal structure and mechanistic investigations on these ions have been published.  

X-ray analysis of ferrocenyldiphenylcarbenium tetrafluoroborate enabled determination of the Fe-C distance of 271.5 pm, and showed that is bent to iron, the angle being 20.7°. It was calculated that the direct interaction of iron electrons with the cationic stabilizes the ferrocenylmethylcarbe-nium ion. It was concluded that the cation can be described as (T -fulvene)iron with a small charge on NMR chemical shifts of the para-carbons in 

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