The structures of dicyclopentadienylsilenes and related compounds

The most stable form, according to MNDO calculations, of the silene (C₅H₅)₂Si is the bis-monohapto isomer, in which the angle (CSiC) is 105.4°, and the conformation is such that the dihedral angles δ (HCSiC) are zero. The bis-pentahapto isomer is of comparable energy, and has minimum energy for a structure of D_5d symmetry: this isomer does not represent a genuine energy minimum. Entirely similar conclusions result from calculations on the isoelectronic (C₅H₅)₂P⁺, but for (C₅H₅)₂Al^?, although the lowest energy form is again (η⁵-C₅H₅)₂Al^? , the (η⁵-C₅H₅)₂Al^? isomer has only C_2v symmetry with angle (XAlX) 151.5°. In the silene derivatives (η¹-C₅H₅)SiCY₂, the unique SiC interaction is a genuine SiC double bond and the molecular skeleton C₂SiCY₂ is planar: however in the isomeric species (η⁵-C₅H₅)₂SiCY₂, which do not represent genuine energy minima, the unique SiC interaction is a long, highly polar bond in a twisted X₂SiCY₂ skeleton in which the X₂SiC and SiCY₂ planes are perpendicular.