(i) H atom transfer is facilitated by the isotropic H 1s orbital that facilitates overlap in the transition state, whereas Me group transfer involves distortion of the more directional sp 3 hybrid (as indicated by NBO analyses on these two systems, Figure and S197). This difference may be enhanced to some extent by the dominant Zn 4s character in the ZnH bond in Int3 DHCZn ; in contrast the Al contribution to the AlMe bond in Int1 DHCAl has more directional sp character (consistent with the relative energy of the 4s vs. 4p orbitals for zinc and the 3s vs. 3p orbitals for aluminum, with the Zn 4p orbitals being relatively high in energy). The residual AlMe interaction is further undermined in TS3 DHCAl compared to the ZnH interaction in TS3 DHCZn . The impact on orbital overlap in the corresponding transition states is shown schematically (see also Figure ).