NEt3 adduct [NacNacAl(Me)(NEt3)] + , 15b + , the reaction proceeds via the dissociative substitution of NEt3 by 2a to form the electrophilically activated adduct Int2 CHAl at +6.8 kcal/mol. Deprotonation of this species by free NEt3 then gives 9a, plus [(Et3N)H] + at +4.2 kcal/mol. This endergonic C-H alumination is followed by an exergonic dehydrocoupling between 8 and [(Et3N)H] + to reform 15b + along with CH4 (G = -15.1 kcal/mol). The overall catalytic cycle is therefore exergonic (G = -10.9 kcal/mol). Calculation of the ligand-assisted protonolysis pathway for Al revealed that while protonation at the NacNac -C is kinetically accessible via TS2 DHCAl at +19.4 kcal/mol, the subsequent CH4 elimination via TS3 DHCAl is much higher at +36.1 kcal/mol. However, in this case direct protonolysis is more accessible via TS1 DHCAl (at +24.9 kcal/mol), with this corresponding to a Me-abstraction involving a near-linear AlCH3HNEt3 unit (Figure (b) top). Binding of Et3N to Al then occurs in a separate step to reform 15b + . It is noteworthy that protonolysis (methane loss) and amine binding occur in separate steps in this pathway due to the accessibility of the amine-free cation, [NacNacAl-Me] + (note, this cation has been previously reported). In contrast, in the zinc system forming the amine-free cation, [NacNacZn] + , results in high energy intermediates (>30 kcal/mol, see Fig. ), precluding a comparable pathway involving direct Zn-H protonolysis followed by a separate amine binding step. Both C-H zincation and alumination reactions are endergonic and the metalated products are trapped through exergonic dehydrocoupling. The computed barrier for the protonolysis of 1 to form 13c + (17.8 kcal/mol) is lower than that for 8 to form 15b + (20.7 kcal/mol), consistent with the relative solution reactivities. Furthermore, the computed barrier for the dehydrocoupling of 8 with [(Et3N)H] + (20.7 kcal/mol) is in good agreement with the experimental value of 21.1 (±1) kcal/mol, and this barrier is lower than that for the protonolysis of 9a, albeit only by 0.2 kcal/mol. The last comparison is clearly subtle, and this extends to the interpretation of the experimental kH/kD KIEs. Thus, the low KIE of 1.2 for the catalytic zincation of 2a (Scheme 5) is consistent with the computed rate-limiting transition state, TS1 CHZn , that corresponds to associative NEt3/2a substitution. In contrast, TS3 CHAl is computed to be rate-limiting for catalytic alumination and this does involve C-H bond cleavage, aligning with the larger KIE of 2.8 (Scheme 7). More generally, however, both cycles involve several high energy transition states of varying character that are close in energy, and for which the relative energetic ordering is functional dependent. The B3PW91 functional was chosen as it performs qualitatively the best over the range of available experimental mechanistic data; a more quantitative interpretation of the computed profiles would be challenging.