In earlier work involving 2a we showed that the syn isomer was more stable than the anti and features a low uptake barrier and so focused on that isomer here when it comes to location of complexes 4. Similarly, the insertion transition state 5 was located via rotation of coordinated propene into the equatorial plane followed by linear transit calculations constraining the Zr-Me---CH(Me)=CH2 distance; frequency calculations were used to confirm the nature of the resulting stationary point, while the imaginary vibration invariably corresponded to formation of the new C-C bond. An issue that arises in connection with 4 and 5 is that both are OSIP with different possible orientations of the cation with respect to the anion. We investigated isomers at HF/3-21G* level of theory, fixing the Zr-Me---CH(Me)=CH2 distance in the case of transition structures 5. Generally, the cation prefers to align along one face of the sheet anion, while the orientation of the cation with respect to this face is usually less important (ΔE < 10 kJ mol -1 ). In this work we deal with the lowest E conformer of those examined, even though one expects several lower E conformers contribute to the free energy of various processes. In the future we plan to see if any of these large systems can be investigated by molecular dynamics; 29 the smallest system investigated here SBIZrMe-μ-Me-6,4 still has over 300 valence electrons and no MM force field for the aluminoxanes exists, precluding QM/MM approaches. For the largest structures featuring the [26,8] - sheet anion, geometry optimizations at M06-2X/TZVP level required typically 1-3 weeks computing time on 40 processors, even if starting from an optimized M06-2X/SV(P) geometry while a frequency calculation at M06-2X/TZVP level also required about a week of CPU time. The insertion energetic data are summarized in Table including that previously reported for 16,6. The insertion barrier ΔG ‡ -qh-tr = 100.3 kJ mol -1 is relative to separated CIP 2a and C3H6. CIP 2b is slightly less reactive (ΔG ‡ -qh-tr = 105.8 kJ mol -1 ) while CIP 2c and 2d are more reactive with ΔG ‡ -qh-tr = 93.7 and 91.0 kJ mol -1 , respectively. All these species are significantly less reactive than either [SBIZrMe][B(C6F5)4] or even SBIZrMe-μ-MeB(C6F5)3 for which the calculated insertion barriers are 58.7 and 77.9 kJ mol -1 , respectively for the first insertion. In contrast, the experimental barriers are comparable; 69-72 vs. 76-79 kJ mol -1 for the 1 st insertion step for borate vs. MAO (at 2400:1 Al:Zr) 2b and borate = 51-57, borane = 59-67, and MAO = 60-66 kJ mol -1 for subsequent insertions. 2b-c [X] ΔE TΔS ‡ -tr TΔS ‡ -qh-tr TΔΔS ‡ [MeB(C6F5)3] - 27.9 -46. Average ± σ --40.6 ± 7.5 -44.4 ± 2.0 7.1 ± 3.8 [a] Energies (kJ mol -1 ) at M06-2X/TZVP level are with respect to CIP 2 + C3H6. [b] TΔΔS ‡ = TΔS ‡ -qh-tr -TΔS ‡ -tr. [c] Average is for the aluminoxanes.