Binding between ground-state aluminum ions and small molecules: A1⁺·(H₂/CH₄/C₂H ₂/C₂H₄/C₂H₆). _n,. Can Al⁺ insert into H₂?

Binding energies and entropies have been measured for the attachment of up to four H₂ ligands and six small hydrocarbons to ground-state Al⁺ ions (¹S, 3s²). 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 (H₂), 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 π-type orbitals prevents donation from the Al⁺ to the σ* orbitals on the ligands. A very detailed theoretical examination of the Al⁺(H₂)_n cluster energetics was also made. The purpose was to investigate the possibility of insertion by the Al⁺ into the H-H bond via σ bond activation, as is found with the isovalent B⁺ ion. The calculations showed that the inserted HAlH⁺ ion is stable but that its formation is endothermic by 10.9 kcal/mol with respect to the separated reactants. The inserted HAlH⁺(H₂)₂ ion, however, appears to be almost isoenergetic with the uninserted Al⁺(H₂)₃ isomeric cluster.