Sigma Bond Activation by Cooperative Interaction with ns² Atoms:  Al⁺ + nH₂

The reactions of Al⁺ + nH₂ to produce AlH₂⁺(H₂)_n-1 have been studied by high-level ab initio electronic structure techniques motivated by the σ bond activation by cooperative interaction observed experimentally and theoretically for the isovalent B⁺ + nH₂ reaction systems. For n = 1, the reaction proceeds stepwise:  first breaking the H₂ bond and forming one AlH bond followed by the formation of the second AlH bond. This process has an activation energy of 85.0 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism through a planar, cyclic transition state where two H₂ bonds are broken simultaneously while two AlH bonds and one new H₂ bond are formed. The activation energy for this process decreases from the n = 1 value to about 55.0 kcal/mol. These two cases are qualitatively very similar to what was observed for B⁺ + nH₂ with the major quantitative differences being that corresponding activation energies were 30−40 kcal/mol lower and reaction energetics were 60−80 kcal/mol more exothermic in the boron systems. For n = 3, no additional activation energy lowering was observed with Al⁺, which contrasts significantly with the behavior observed with B⁺. This difference is rationalized in terms of the special ability of boron to form strong three center−two electron bonds.