BINDING BETWEEN GROUND-STATE ALUMINUM IONS AND SMALL MOLECULES - AL-DOT(H-2(CENTER)CH4/C2H2/C2H4/C2H6)(N) - CAN AL+ INSERT INTO H-2/

Binding energies and entropies have been measured for the attachment o f 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 wit h analogous transition metal ion or the isovalent boron ion systems. B ond 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 o f 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 donati on to the metal ion and also reduces the Al+/ligand attraction. Finall y, the lack of low lying, occupied pi-type orbitals prevents donation from the Al+ to the sigma orbitals on the ligands. A very detailed th eoretical examination of the Al+(H-2)(n) cluster energetics was also m ade. The purpose was to investigate the possibility of insertion by th e Al+ into the I-I-I-I bond via sigma bond activation, as is found wit h the isovalent B+ ion. The calculations showed that the inserted HA1H (+) ion is stable but that its formation is endothermic by 10.9 kcal/m ol 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.