Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)(2) (L = Cp, 1,2-Me2Cp, Me2Cp, Me5Cp; L-2 = Me2Si(Me4Cp)((BuN)-Bu-t); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)(3) were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3+CH3B(C6F5)(3)(-) contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, Delta H = -23.1(3), -24.3(4), and -36.7(5) kcal mol(-1) for L = Cp, Me2Cp, and Me5Cp, respectively. For M = Hf and L = 1,2-Me2Cp, Delta H = -20.8(5) kcal mol(-1). "Constrained geometry" complexes (L-2 = Me2Si(Me4Cp)((BuN)-Bu-t)) exhibit similar exothermicities, with Delta H = -22.6(2), -23.9(4), and -19.3(6) kcal mol(-1) for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane M:Al = 1:50) are less exothermic, with Delta H = -10.9(3) and -8.9(4) kcal mol(-1) for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)(2)M(CH3)(2) (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)(2)BAr (Ar = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2Cp)(2)MCH3+ CH3B(C6F5)(2)Ar- ion pairs are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, Delta H = -18.7(7) and -15.2(8) kcal mol(-1) with Delta S = -42(2) and -35(3) e.u.; for Ar = Ph, Delta H = -14.8(8) and -13.3(6) kcal mol(-1) with Delta S = -31(2) and -39(2) e.u.; for Ar = 3,5-C6H3Me2, Delta H = -10.8(6) and -12.7(5) with Delta S = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (Delta H double dagger = 2-6 kcal mol(-1)) while ion-pair separation/ recombination processes an greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)(2)MCH3+ CH3B(C6F5)(2)Ar- complexes measured in toluene solution (25 degrees C, 1 atm) follow a trend in metal (Zr > Hf) as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph similar to 3,5-C6H3Me2). Polymerization activities correlate roughly with MCH3+ 13C NMR chemical shifts and enthalpies of methide abstraction.