The analysis of the H2 activation by cAACs and aAACs indicates that the activation of hydrogen molecules results from the primary interaction between lone-pair electrons of carbene and the orbitals of H2. This will form two hydride-like hydrogen atoms attaching to the carbon centre. The covalent main group activation is similar to the transition metal-mediated oxidative addition. The orbital interaction of divalent main-group species and H2 bond occurs in a side-on fashion and is characterised by two categories: s-donation from the s orbital to the π*-orbital of appropriate symmetry and s-donation from the centre atom of carbene to the H2 s* orbital. 2