These factors were explored further by computing the ligand-assisted dehydrocoupling step in a series of model complexes ( A related effect is observed in the zinc system on varying the amine spectator ligand: replacing NEt3 with smaller amines (NMe3 and NH3, compare Entries 8 and 9 to Entry 1) decreases the distortion in the precursor (higher ) and leads to higher barriers to H2 loss. It is also notable that the closest Al/Zn pairs, in terms of steric demand of the spectator ligand (entries 7 and 8 and entries 5 and 9), have extremely similar barriers to H2 loss (11.3 vs. 11.4 and 14.1 vs. 13.9 kcal/mol), indicating that in these systems the steric bulk of the spectator ligand plays a greater role in determining the barrier to dehydrocoupling than the identity of the metal and the nature of the M-Y interaction. It is notable that the kinetically most accessible dehydrocoupling reactions (Entries 1 and 7) involve H2 loss in the presence of the bulkiest spectator ligand explored; the latter pre-organizes the system to such an extent that  actually increases (i.e., the distortion decreases) in the transition state. This significant effect (up to 5 kcal/mol G ‡ in the series studied herein) has implications for the design of NacNacM catalysts, as it suggests that increasing the steric bulk around the metal center can actually be desirable for lower barrier protonolysis processes via a ligand-assisted mechanism.