Calculations have been carried out on the reaction between the olefin polymerization precatalyst (1,2-Me2Cp)2ZrMe2 (P) and a number of Lewis acids (A) to form the methide-bridged (contact) ion pair (1,2-Me2Cp)2ZrMe(μ-Me)A (I; P + A → I + ΔHipf). This is the first step in the activation of P to becoming an olefin polymerization catalyst. The calculated enthalpies of formation (ΔHipf) for I were as follows (A, ΔHipf in kcal/mol): B(C6F5)3 (1a), −23.8; B(C6F5)2(C6H3F2) (1b), −21.5; B(C6F5)2(C6H5) (1c), −18.3; B(C6F5)2(C6H3(CH3)2 (1d), −18.0; B(C6H5)3 (1e), −6.7; B(C10F7)3 (1f), −25.8; (MeAlO)6 (2a), −15.9; (MeBO)6 (2b), −22.3; AlMe3 (2c), −8.1; Al(C6F5)3 (2d), −30.8. The charge separation between the (1,2-Me2Cp)2ZrMe+ and AMe- fragments in I was calculated for all A, and it was found that the charge separation as well as −ΔHipf increases through the series 1e, 1c, 1b, and 1a with the number of fluorine atoms. A good activating Lewis acid (A) has the equilibrium shifted strongly from P and A toward I, and this is the case for all A except 1e and 2c. Also considered was the complete dissociation in solution (toluene) of I into the counterions [(1,2-Me2Cp)2ZrMe]+ and AMe- with the dissociating enthalpy ΔHips as well as the formation from I of the solvent-separated ion pair (S = toluene) [(1,2-Me2Cp)2ZrMe]+−S−[AMe]- with the reaction enthalpy ΔHss. The two types of separation processes have both been postulated as the second and final steps in the activation of P. The calculated values are as follows (A, ΔHips and ΔHss in kcal/mol): 1a, 38.0, 18.7; 1f, 43.6, 18.9; 2a, 57.0, 32.4; 2b, 46.9, 35.3; 2c, 69.2, 35.3; 2d, 48.3, 20.6. It is concluded that the formation of [(1,2-Me2Cp)2ZrMe]+−S−[AMe]- is the more likely separation process. Consideration has also been given to the influence of solvent polarity on the separation process, with S = toluene, chlorobenzene, and 1,2-dichlorobenzene. Finally discussed are ΔHips and ΔHss for the ion pair [(1,2-Me2Cp)2ZrMe]+[A]- (III‘); A- = B(C6F5)4- (3a), Al(C6F5)4- (3b), [(C2B9H11)2Co]- (3c), and {tBuCH2CH[B(C6F5)2]H}- (3d)), where III‘ is formed from the reaction of P with the activator [CPh3+][A-]. Here the calculated values are as follows (A-, ΔHips and ΔHss in kcal/mol): 3a, 22.1, −4.2; 3b, 26.2, 0.7; 3c, 34.9, 7.5; 3d, 26.7, −0.5. It is found that III type ion pairs are easier to dissociate than I held together by a methide bridge.