Sigma bond activation by cooperative interaction with ns(2) atoms: Al++nH(2)

The reactions of Al+ + nH(2) to produce AlH2+(H-2)(n-1) have been studied b y high-level ab initio electronic structure techniques motivated by the a b ond activation by cooperative interaction observed experimentally and theor etically for the isovalent B+ + nH(2) reaction systems. For n = 1, the reac tion proceeds stepwise: first breaking the Hz bond and forming one AlH bond followed by the formation of the second AlH bond. This process has an acti vation energy of 85.0 kcal/mol. For a = 2, the reaction proceeds via a peri cyclic mechanism through a planar, cyclic transition state where two H-2 bo nds are broken simultaneously while two AIH bonds and one new H-2 bond are formed. The activation energy for this process decreases from the a 1 value to about 55.0 kcal/mol. These two cases are qualitatively very similar to what was observed for B+ + nH(2) with the major quantitative differences be ing that corresponding activation energies were 30-40 kcal/mol lower and re action 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 dif ference is rationalized in terms of the special ability of boron to form st rong three center-two electron bonds.