STRUCTURES AND MOLECULAR-PROPERTIES OF ST ERICALLY OVERCROWDED MOLECULES .36. (E)-1,1,1,4,4,4-HEXAKIS(TRIMETHYLSILYL)-2-BUTENE [(H3C)3SI]3C-HC=CH-C[SI(CH3)3]3 AND BIS[TRIS(TRIMETHYLSILYL)SILYL]ETHIN [(H3C)3SI]3SI-C-EQUIVALENT-TO-C-SI[SI(CH3)3]3

The single crystal structure of an ethylene derivative, in which the t wo substituent half-shells are separated by a C=C spacer of about 403 pm length, shows C...C distances of 404, 415 as well as 421 pm between non-adjacent and of 352 pm between adjacent methyl groups, i.e. close to or within their van der Waals radii sum of 400 pm. The PE-spectros copically determined vertical first ionization energies of both compou nds, 7.78 eV and 7.71 eV, are each the lowest ones reported so far for disubstituted ethylenes and acetylenes. Despite of irreversible cyclo voltammetric oxidation potentials, the radical cation of the six-fold R3Si-substituted 2-butene can be generated by SbCl5/H2CCl2 and charact erized below 230 K by its ESR spectrum.