Crystals of hexa- tert -butyldisilane, C 24 H 54 Si 2 , undergo a reversible phase transition at 179 (2) K. The space group changes from Ibca (high temperature) to Pbca (low temperature), but the lattice constants a , b and c do not change significantly during the phase transition. The crystallographic twofold axis of the molecule in the high-temperature phase is replaced by a noncrystallographic twofold axis in the low-temperature phase. The angle between the two axes is 2.36 (4)°. The centre of the molecule undergoes a translation of 0.123 (1) Å during the phase transition, but the conformation angles of the molecule remain unchanged. Between the two tri- tert -butylsilyl subunits there are six short repulsive intramolecular C—H...H—C contacts, with H...H distances between 2.02 and 2.04 Å, resulting in a significant lengthening of the Si—Si and Si—C bonds. The Si—Si bond length is 2.6863 (5) Å and the Si—C bond lengths are between 1.9860 (14) and 1.9933 (14) Å. Torsion angles about the Si—Si and Si—C bonds deviate by approximately 15° from the values expected for staggered conformations due to intramolecular steric H...H repulsions. A new polymorph is reported for the crystal structure of 1,1,2,2-tetra- tert -butyl-1,2-diphenyldisilane, C 28 H 46 Si 2 . It has two independent molecules with rather similar conformations. The Si—Si bond lengths are 2.4869 (8) and 2.4944 (8) Å. The C—Si—Si—C torsion angles deviate by between −3.4 (1) and −18.5 (1)° from the values expected for a staggered conformation. These deviations result from steric interactions. Four Si—C( t -Bu) bonds are almost staggered, while the other four Si—C( t -Bu) bonds are intermediate between a staggered and an eclipsed conformation. The latter Si—C( t -Bu) bonds are about 0.019 (2) Å longer than the staggered Si—C( t -Bu) bonds.
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