The thermodynamic and structural characteristics of Al(C6F5)(3)-derived vs B(C6F5)(3)-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)(3) or 1.0 or 2.0 equiv of Al(C6F5)(3) with raC-C2H4(eta(5)-Ind)(2)Zr(CH3)(2) (rac-(EBI)Zr(CH3)(2)) yields rac-(EBI)Zr(CH3)+H3CB(C6F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[(HCAl)-C-3(C6F5)(3)](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al(CH3)(bridging) bond lengths of 1b (2.505(4) angstrom and 2.026(4) angstrom, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)(3)-derived ion pairs. Ion pair formation enthalpies (Delta H-ipf) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1 (1) kcal mol(-1) (1b -> 1c), indicating Al(C6F5)(3) to have significantly less methide affinity than B(C6F5)(3). Analogous experiments with Me2Si(eta(5)-Me4C5)(t-BuN)Ti(CH3)(2) indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are Delta H-ce(double dagger) = 22(1) kcal mol(-1), S-ce(double dagger) = 8.2(4) eu, Delta H-ae(double dagger) = 14(2) kcal mol(-1), and Delta S-ae(double dagger) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant (similar to 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives Delta H-ce(double dagger) approximate to 22 kcal mol(-1) and Delta H-ae(double dagger) approximate to 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.