A Neutral Silicon(II) Half-Sandwich Compound

Metathesis reaction of a dilithio borole dianion, a cyclic π-ligand isoelectronic to ubiquitous cyclopentadienyls, with two equivalents of “silicocenium” cation [Cp*Si]+ as a source of low-valent Si­(II), cleanly gives a borole half-sandwich π-complex of Si­(II) and silicocene. The resulting half-sandwich complex is a neutral isoelectronic analogue to the iconic silicocenium cation and features the rare structural motif of an apical silicon­(II) atom with an energetically high lying lone pair of electrons that is shown to be accessible for coordination chemistry toward tungsten carbonyl. Protonation at the Si­(II) atom with [H­(OEt2)2]­[Al­{OC­(CF3)3}4] induces formal oxidation, and the compound rearranges to incorporate the Si atom into the carbocyclic base to give an unprecedented cationic 5-sila-6-borabicyclo[2.1.1]­hex-2-ene. This rearrangement is accompanied by drastic changes in the 11B and 29Si NMR chemical shifts.