[4d], which was also accepted in a more recent, comprehensive study of MAO. We investigated this possibility using the [7,4] -anion and the neutral 6,4 sheet. The 6,4 neutral has a Lewis acidic OAlMe2 site and it forms a stable donor-acceptor adduct with the [7,4] - anion (Δ ‡-tr = -14.5 kJ mol -1 in toluene continuum), resulting in a hybrid [13,8] -anion of almost equivalent molar mass to the [16,6] -anion, thus allowing a comparison of reactivity independent of sheet size. In this hybrid anion the formal anionic moiety is a sheet-OAlMe2-μ-Me-AlMeO2-sheet group, with a linear Al-μ-Me-Al geometry, which is partially shielded by each sheet from interacting easily with a Zr centre. However, it is still possible for the SBIZr + Me cation to bind to this hybrid anion through another, more distant O2AlMe2 group and CIP 2e, an isomer favoured by electrostatics and located in the same manner as 2b and 2d, is shown in Figure . The formation of the syn 4e π complex from CIP 2e (Figure ) is expected to be freely reversible with a low barrier based on their similar ΔG-qh-tr values (Table ) and studies of the uptake barrier in the case of ion-pair 2a. In fact, formation of 4e features a large electronic stabilization compared with 4a (or 4b) and by more than 20 kJ mol -1 (Table ). The cation in 4e is involved in dispersive, non-bonded interactions with the 7,4 sheet (on the right hand side of the anion, Figure ) but, notice that the formal anionic moiety is quite far removed from the metal centre in this stable intermediate. The insertion barrier featuring this hybrid anion is much lower than that seen for the [26,8] -or [16,6] -anion, and based on ΔG ‡ -qh-tr = 58.4 kJ mol -1 is competitive with that seen for [SBIZrMe][B(C6F5)4]! After insertion, the kinetic product features the γ-CH agnostic structure 6e but notice how the cation has reoriented with respect to the anion with the formal anionic moiety now behind the Me2Si bridge and unable to easily interact with the metal centre. We expect further insertion involving this intermediate would be facile and wonder whether binding and insertion of monomer would be competitive with collapse to form homologated CIP 2e-i-Bu and then ultimately, reversible dissociation to generate 2b-i-Bu (effectively dormant in comparison to 2e-i-Bu) and 6,4. The involvement of species like 2e vs. 2b might account for intermittent vs. continuous propagation behavior invoked for some metallocene catalysts. dispersive interactions in CIP 2 vs. OSIP 3. The results are summarized in Table . Formation of ion-pair 2c is more favorable than 2e (entry 1 vs. 2). Formation of 2e is favored by both ΔE and ΔH-qh but is accompanied by a large, -TΔS-qh-tr term as the forward reaction is termolecular vs. bimolecular in the case of 2c. However, direct formation of ion-pair 3e is more favorable than formation of 3b (entry 3 vs. 4), despite the higher -TΔS-qh-tr. Dissociation of ion-pairs 3 into 2 and monomeric Me3Al is unfavorable for all species, though theory would predict ion-pair 3b to be the most dissociated in solution (entries 6-8). This along with the insertion barriers (vide supra) suggests that of the three anions, the [7,4] -anion is most strongly coordinating.