Sigma Bond Activation by Cooperative Interaction with <i>n</i>s<sup>2</sup> Atoms:  Al<sup>+</sup> + <i>n</i>H<sub>2</sub>

The reactions of Al<sup>+</sup> + <i>n</i>H<sub>2</sub> to produce AlH<sub>2</sub><sup>+</sup>(H<sub>2</sub>)<i><sub>n</sub></i><sub>-</sub><sub>1</sub> 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<sup>+</sup> + <i>n</i>H<sub>2</sub> reaction systems. For <i>n</i> = 1, the reaction proceeds stepwise:  first breaking the H<sub>2</sub> 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 <i>n</i> = 2, the reaction proceeds via a pericyclic mechanism through a planar, cyclic transition state where two H<sub>2</sub> bonds are broken simultaneously while two AlH bonds and one new H<sub>2</sub> bond are formed. The activation energy for this process decreases from the <i>n</i> = 1 value to about 55.0 kcal/mol. These two cases are qualitatively very similar to what was observed for B<sup>+</sup> + <i>n</i>H<sub>2</sub> 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 <i>n</i> = 3, no additional activation energy lowering was observed with Al<sup>+</sup>, which contrasts significantly with the behavior observed with B<sup>+</sup>. This difference is rationalized in terms of the special ability of boron to form strong three center−two electron bonds.