STRUCTURES OF STERICALLY OVERCROWDED OR CHARGE-PERTURBED MOLECULES .28. [(H3C)(3)SI]C-3-C-C-C[SI(CH3)(3)](3) - A PI-SYSTEM CONCEALED BY A HYDROCARBON VEIL

The kinetically shielded di(tris(trimethylsilyl)methyl)acetylene crystallizes in the triclinic space group P (1) over bar with 1 molecule per unit cell in rather dense lattice packing. Its staggered skeletal conformation of close to D-3d symmetry represents a sterically advantageous cogwheel gearing of the inner methyl groups of the half-shells of both substituents. The shortest nonbonded C...C distances of 400 pm between them equal the sum of the van der Waals radii of two H3C groups. A space-filling representation of the molecular structure shows the acetylenic pi-system to be almost completely covered by the peripheral HC skin, in keeping with its unusual unreactivity, e.g. towards catalytic hydrogenation. The spatial and electronic effects of the bulky substituents are discussed in terms of structural comparisons with chemically related compounds such as [(H3C)(3)Si]Si-3-Si[Si(CH3)(3)](3), [(H3C)(3)C]Si-3-Si[C(CH3)(3)](3) or [(H3C)(3)Si]C-3-C6H4-C[Si(CH3)(3)](3), which also adopt staggered conformations of their sterically congested [(H3C)(3)X]Y-3-half shells along substituent axes of C-3-symmetry at different distances Y...Y.