Ab Initio Investigations of Structure and Stability of [R₃SiAl]₄, R = H, Me, tBu

The second largest molecule so far dealt with by ab initio methods is the tetrameric Al 1 compound [ t Bu 3 SiAl] 4 . The calculations yielded structure parameters that are in agreement with those of related compounds, and showed that, above all, the lack of π backbonding in the monomer explains why the tetramer is so stable that it can be sublimed at 180°C without decomposition. In contrast to H 3 SiAl and Me 3 SiAl, for t Bu 3 SiAl the SiAl distance is only slightly shortened on tetramerization, which could indicate slight repulsion between the t Bu 3 Si groups in the tetramer. As a result of the nonpolar surface of the tetramer, a high degree of disorder in the crystal is to be expected—a possible explanation for the difficulties encountered in the structure determination.