Hypercoordination to a saturated carbon atom

Recently, the first heptacoordinate disilane [(MesCS 2 ) 2 MeSi] 2 1 was synthesized and shown to have a unique structure by X-ray analysis. Each of the silicon atoms is surrounded by three nonbonded thiobenzoylthio groups. We theoretically investigated the origins of the unusual geometry of 1 containing multiple hypercoordinations. The model compound [(HCS 2 ) 2 MeSi] 2 2a shows both the H-CSi-Si eclipsed conformation and the C-SiSi-C eclipsed conformation. The dihedral angle of H-CSi-Si is calculated to be 24.6° at the RHF/6-31G* level (25.7° at the RHF/LANL2DZ+p level). Each of the carbon atoms of the methyl groups is also surrounded by three nonbonded thioformylthio groups. After the bond model analyses, we found that hypercoordination to the saturated carbon atom is essential for this unique geometry, as confirmed by scrutinizing the X-ray data of 1 .