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 cryst allizes in the triclinic space group P (1) over bar with 1 molecule pe r unit cell in rather dense lattice packing. Its staggered skeletal co nformation of close to D-3d symmetry represents a sterically advantage ous cogwheel gearing of the inner methyl groups of the half-shells of both substituents. The shortest nonbonded C...C distances of 400 pm be tween them equal the sum of the van der Waals radii of two H3C groups. A space-filling representation of the molecular structure shows the a cetylenic pi-system to be almost completely covered by the peripheral HC skin, in keeping with its unusual unreactivity, e.g. towards cataly tic hydrogenation. The spatial and electronic effects of the bulky sub stituents are discussed in terms of structural comparisons with chemic ally related compounds such as [(H3C)(3)Si]Si-3-Si[Si(CH3)(3)](3), [(H 3C)(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 congest ed [(H3C)(3)X]Y-3-half shells along substituent axes of C-3-symmetry a t different distances Y...Y.