Decamethylsilicocene — The first stable silicon(II) compound: Synthesis, structure, and bonding

Reaction of dichlorobis(pentamethylcyclopentadienyl)silane with naphthalene‐lithium, ‐sodium, or ‐potassium leads to elemental silicon and to decamethylsilicocene, (Me 5 C 5 ) 2 Si ( 4 ). Compound 4 is formed as the only product in the reduction of dibromobis(pentamethylcyclopentadineyl)silane with anthracene‐potassium. 4 is a thermally stable, colorless, air‐sensitive π‐complex. Its NMR spectra are typical for a group 14 metallocene; the 29 Si‐NMR signal appears at very high field strength (δ = −398 ppm). CV and MS data of 4 prove the instability of the (Me 5 C 5 ) 2 Si ⊖ ion, which easily looses a Me 5 C 5 radical. X‐ray crystallographic studies show the presence of two geometrical isomers, 4a and 4b , in the monoclinic unit cell. Isomer 4a is isotypical with decamethylferocene, isomer 4b possesses the expected bent‐metallocene‐type structure. Space‐filling models indicate the interplane angle in 4b to be of the largest possible value. Due to GED studies, 4 has a bent‐metallocene‐type structure in the gas phase. The He(I) PE spectrum of 4 is compared with those of the heavier homologues (Me 5 C 5 ) 2 Ge ( 6 ), (Me 5 C 5 ) 2 Sn ( 7 ), and (Me 5 C 5 ) 2 Pb ( 8 ). A strong shift to higher energy of the band assigned to the group 14 element lone pair is observed in going from 8, 7 , or 6 to 4 . Calculations have been carried out for structural models of the parent silicocene, (H 5 C 5 ) 2 Si, on the basis of the MNDO procedure and of the HF theory (STO‐3G and STO‐3G* basis sets). The calculated geometrical parameters and orbital energies are compared with those derived from the GED experiment and from the PE spectrum. The bathochromic shift in the electronic absorption spectra going from 4 to 6 – 8 is explained on the basis of MNDO calculations for the parent metallocenes.