Binding between ground-state aluminum ions and small molecules:: Al+•(H2/CH4/C2H2/C2H4/C2H6)n.: Can Al+ insert into H2?

Binding energies and entropies have been measured for the attachment of up to four Hz ligands and six small hydrocarbons to ground-state Al+ ions (S-1, 3s(2)). Bond energies are typically very weak compared with analogous transition metal ion or the isovalent boron ion systems. Bond energies for the first ligand addition to Al+ are 1.4 (H-2), 6.1 (CH4), 9.3 (C2H6). 14.0 (C2H2), and 15.1 kcal/mol for C2H4 The origin of the weak bonding lies primarily in the large, repulsive 3s orbital, which prevents close approach by the ligands. In addition, the lack of low-energy acceptor orbitals on the Al+ ion minimizes electron donation to the metal ion and also reduces the Al+/ligand attraction. Finally, the lack of low lying, occupied pi-type orbitals prevents donation from the Al+ to the sigma* orbitals on the ligands. A very detailed theoretical examination of the Al+(H-2)(n) cluster energetics was also made. The purpose was to investigate the possibility of insertion by the Al+ into the I-I-I-I bond via sigma bond activation, as is found with the isovalent B+ ion. The calculations showed that the inserted HA1H(+) ion is stable but that its formation is endothermic by 10.9 kcal/mol with respect to the separated reactants. The inserted HAIH(+)(H-2)(2) ion, however, appears to be almost isoenergetic with the uninserted Al+(H-2)(3) isomeric cluster.