The solid-state structure of [NacNacAl(Me)(DMT)][B(C6F5)4], 15a (Figure ), confirmed the formulation with the four coordinate aluminum center in a distorted tetrahedral geometry with metrics comparable to other four-coordinate NacNacAl(Me)-based complexes. Steric effects in 15a appear minimal as indicated by the Al-NDMT distance in 15a (Al1-N3 2.010( )) being comparable to that in other cationic four-coordinate Al-N(Ph)Me2 complexes that have ligands with less bulky substituents. As 15b did not afford single crystals suitable for X-ray diffraction analysis, the structure of the cationic component was calculated (vide infra for detailed discussions of computational results). This calculated structure possessed closely comparable metrics to the structure of 15a indicating minimal differences to the geometry around aluminum on changing the amine from DMT to Et3N. Seeking definitive evidence for backbone protonation, the reaction of NacNacZnEt with the strong Brønsted acid HNTf2 was investigated. This produced a new compound with a singlet at δ 4.29 in 1 H NMR spectrum, integrating to 2H and assigned to backbone C methylene protons (Fig. ). This is consistent with the formation of compound 19, a conclusion supported by the 13 C{ 1 H} NMR spectrum containing a resonance for a methylene () carbon at δ 45.6. H unit while notable does not unambiguously confirm that this is an on-cycle process. Reversible C protonation followed by direct protonolysis of Zn-H is also feasible, therefore DFT studies were performed to investigate the mechanism further (vide infra).