Highly Asymmetric Coordination in Alkenes:  Gas-Phase Structures of trans-1,2-Dichloro-1,2-disilylethene and 1-Bromo-1-silylethene

The molecular structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene have been determined by gas-phase electron diffraction (GED) and ab initio molecular orbital calculations (MP2/6-311G*). Both compounds were found to have highly asymmetric coordination around the carbon atoms with [ab initio (r e)/GED (r a)] CCCl [117.0/117.0(2)°] and CCSi [126.2/128.1(1)°] in the C 2 h structure of trans-1,2-dichloro-1,2-disilylethene and CCBr [119.2/120.7(4)°] and CCSi [125.0/125.0(4)°] in the C s structure of 1-bromo-1-silylethene. Other important structural parameters for trans-1,2-dichloro-1,2-disilylethene are CC [135.2/134.5(3) pm], CSi [189.4/187.9(2) pm], and CCl [175.1/174.9(1) pm], and CC [134.2/133.4(2) pm], C−Si [187.8/187.2(3) pm], and C−Br [191.3/191.0(3) pm] for 1-bromo-1-silylethene. Further ab initio calculations were carried out on CH2CRX and trans-(CRX)2 (R = SiH3, CH3, or H; X = H, F, Cl, or Br) to gauge the effects of electron-withdrawing and electron-donating groups on the structures. They reveal some even more distorted structures. The asymmetric appearance of these molecules can largely be accounted for by valence shell electron pair repulsion theory.