Electronic and Geometrical Structures of Dialuminoxane, Diboroxane, and Their Sulfur Analogues: Ab Initio Study of H₂X‐Y‐XH₂ (X = Al, B; Y = O, S) Compounds

The structures of H 2 XYXH 2 compounds (X = B, Al; Y = O, S) have been studied by means of ab initio calculations at the MP4/6‐311 G ** /MP2/6‐311G ** level. The potential energy surface (PES) of the aluminoxane species H 2 AlOAlH 2 is rather flat: the energy differences between the various located extrema are less than 1.5 kcal mol −1 , and this shows that the observed geometrical preference is not due to electronic factors. For the sulfur analogues H 2 XSXH 2 (X = B, Al), three minima are located on the PES: a planar C 2v structure, a C s structure in which one hydrogen atom bridges the two X atoms, and a C 2v structure with two bridging hydrogen atoms. For H 2 AlSAlH 2 , the C s structure is the most stable, and the planar C 2v and the dibridged structures lie 3.2 and 6.7 kcal mol −1 higher, respectively. Qualitatively similar results are obtained for H 2 BSBH 2 : the C s and planar C 2v structures are nearly isoenergetic, and the dibridged minimum lies 19.4 kcal mol −1 above the planar C 2v minimum. These results are rationalized by analyzing the ability of these systems to bend, depending on the nature (oxygen or sulfur) of the central Y atom. The conjugation in these species is also discussed, and calculations on model systems H 2 XYH (X = B, Al; Y = O, S) are presented.