The [FeCp2]+ and [CoCp2]+ salts are the first examples of metalloorganic complexes where the proton donor eta5-Cp shows both "proper" and "improper" hydrogen bonds when it is involved in intermolecular bonding with different anionic proton acceptors. Blue or red shift of the C-H stretching frequency is the clear experimental evidence.
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Diana et al. (2004) studied this question.
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