Inversion of stereoselectivity in a metallocene catalyst

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.