Classical planar doubly bonded Si-substituted silenes - a boundary system between olefins and heavier Group 14 analogs Geometric and electronic structures, rotational barriers about the Si=C double bond, and comparison with the analogs and isoelectronic phosphenes, further evidence against C-ylides of silicon

An ab initio study of a number of isostructural ethenes, silenes, and germenes at the MP4/6-311G(d)HMP2/6-31G(d) +ZPE level of theory showed that R2Si=CH2 silenes are the last classical planar doubly bonded system because unlike the heavier Group 14 analogs electronegative substituents do not disturb a planar geometry, shorten and weaken the Si C double bond. The calculations of the potential energy profiles and the rotational barriers of isoelectronic silene and phosphene as well as phosphorane are in favor of silenes to be more like phosphenes rather than phosphoranes. The rotational barriers decrease as more electronegative substituents are attached to the Group 14 atom. For ethenes, silenes, and germenes the maximal effect is observed for fluorine substitution. Fluorine does not affect the rotational barrier in phosphenes. A thermochemical approach based on the strain energies and 2 +2 cycloreversion enthalpies was used to estimate the difference between the E=C (E = C, Si, Ge, P) sigma- and pi-bond energies in elementaalkenes. The Bader analysis of the electron density distribution results in a covalent and highly polar double bonds whose polarity decreases in the order: silenes > phosphenes > germenes. (C) 2003 Elsevier B.V. All rights reserved.