Molecular orbital study of the bridging dioxygen ligand in [Al2(CH3)6(O2)]–

Abstract

A recent X-ray structure determination of [Al₂(CH₃)₆(O₂)]^– shows a new bridging co-ordination mode of the dioxygen ligand, with O–O distance and ν(O–O) frequency close to the values observed in solid Na₂O₂. This could suggest classifying the dinuclear aluminium compound as a peroxo complex. On the basis of results obtained from ab initio molecular orbital–linear combination of atomic orbital calculations, a different description of the electronic structure of the title compound is proposed. Despite the fact that the co-ordinated O₂ ligand carries a small negative charge (0.41 electrons), the computed O–O bond length and harmonic frequency are close to the theoretical values expected for the gaseous O₂^– species. As a consequence, the classification of the dioxygen complex as a peroxo or superoxo complex is of little help in elucidating the real electronic structure of the dioxygen ligand. The electronic mechanism of bonding of the dioxygen to an s–p element like aluminium is markedly different from that occurring in transition-metal compounds, despite some similarities in charge distribution and O–O bond length.