Inversion of stereoselectivity of a particular metallocene (Me2Si(Flu) (N‐t‐Bu)ZrCl2) (Me: methyl, Flu: fluorenyl, t‐Bu: tert‐butyl) from syndiospecific into isospecific was observed by changing the cocatalyst from methylaluminoxane (MAO) to [Ph3C]+ [B(C6F5)4]−/AliBu3 (iBu:isobutyl). The change of the solvent‐separated ion pair (in case of MAO as cocatalyst) into the contact ion pair (in case of the more polar and less bulky borate anion) was proposed as the plausible explanation of this phenomenon.
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Shiomura et al. (1996) studied this question.
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