Probing the reactivity of aluminum(I) compounds:: The reaction of pentamethylcyclopentadienyl-aluminum, Al[C5(CH3)5], monomers with dihydrogen in a solid Ar matrix to give the new aluminum hydride molecule H2Al[C5(CH3)5]

The photoinduced reaction of AlCp* [Cp* = C-5(CH3)(5)] monomers with H-2 in an Ar matrix at 12 K was followed with IR spectroscopy. The experimental results allied with detailed quantum chemical calculations show that the product of this reaction is the new aluminum hydride species H2AlCp*. The properties of this species are investigated and compared with those of related known monomeric hydrides such as AlH3, H2AlNH2, and HAlCl2. The comparison between the reactivities of species such as AlCl and AlCp* is of importance for a better understanding of the bonding properties in AlCp*. The possibility for pi-back-donation in the product of the reaction between AlCp* and H-2, H2MCp*, certainly affects chemical properties such as the Lewis acidity. The differences between the wavenumbers for the nu(Al-H) stretching modes of H2AlCp* and H2AlCl show that the influence of the Cp* group is significant.