While both the CH zincation and alumination reactions follow the same general pattern of sequential CH activation/dehydrocoupling, these two processes do differ in the details. In some cases, this reflects the valency of the metal centers; NEt3/2a substitution is an associative substitution at the 3-coordinate Zn in 13c + , whereas starting from the 4-coordinate Al complex 15b + substitution proceeds via a dissociative process. Furthermore, direct protonolysis of the ZnH bond in 1 involves approach of [(Et3N)H] + from above the coordination plane in TS4 DHCZn , with concerted Zn-H bond cleavage and Zn-NEt3 bond formation necessary to avoid the intermediacy of the high energy two coordinate [NacNacZn] + cation. In contrast, such an approach is not found for the direct protonolysis of the AlMe bond in 8 and instead [(Et3N)H] + engages in an essentially linear Me abstraction via TS1 DHCAl . This is presumably due to the higher coordination number (four in 8 vs. three in 1) and the fact that the amine-free cation [NacNacAl-Me] + is energetically accessible. Notably, the barrier to ligand-assisted H2 loss from Int2 DHCZn (G ‡ = 9.2 kcal/mol) is much lower than that for ligandassisted CH4 loss from Int1 DHCAl (G ‡ = 21.7 kcal/mol). This significant difference combined with the ubiquity of NacNacM complexes in chemistry 32 means the factors impacting the barrier to dehydrocoupling via the ligand assisted pathway warrant further discussion. Two main factors appear to underpin this barrier: