Ylenes in the M^II→Si^IV (M=Si, Ge, Sn) coordination mode

The reaction of the methimazolyl (mt, i.e., 2-mercapto-1-methylimidazolide) substituted silane Si(mt)₄ with SnCl₂ and GeCl ₂ in dioxane affords the paddlewheel-shaped complexes [ClSi(μ-mt)₄MCl] (M=Sn (1) and Ge (2), respectively). These compounds represent the first crystallographically characterized hexacoordinate silicon complexes comprising a Sn or Ge atom in the Si coordination sphere. An attempt to synthesize the related silicon compound 3 [ClSi(μ-mt) ₄SiCl] instead afforded the trisilane [ClSi(μ-mt) ₄Si-SiCl₃] (3a), which provides the first crystallographic evidence for the feasibility of oligosilanes with adjacent hexacoordinate Si atoms. One of the hexacoordinate Si atoms of 3a features the unprecedented (Si₂S₄)Si skeleton. Natural bonding orbital (NBO) analyses of compounds 1, 2, 3a (and the target compound 3) revealed characteristics of M^II→Si^IV (for 2 and 3) or M^I→Si ^IV (for 3a) dative bonding in the systems with M=Si and Ge, whereas compound 1 exhibits a covalent Sn^III-Si^III bond. The first structurally evidenced oligosilane with two hexacoordinate Si atoms exhibits electronic features of a Cl₃SiSi→SiCl dative bond, as shown by a natural bonding orbital analysis. Whereas the same features apply to a related methimazolide-bridged ClGe^II→Si^IVCl complex, the corresponding Sn-Si system exhibits pronounced features of a covalent Sn ^III-Si^III bond.