Will an η³-Si₃H₃ ligand form sandwich compounds with main group elements?

Abstract

η³-Si₃H₃ sandwich compounds 5 and 6, with classical and H-bridged ligands, respectively, having the main group elements boron and carbon as central atoms are minima at B3LYP/6-311++G(2d,2p). The stability of these systems is assisted by transfer of charge from the ligands to the central atom and is reversed from that of cyclopentadienyl sandwiches. The C and B containing pyramidal complexes 7, containing both a η³-Si₃H₃ and a μ²-Si₃H₃ ligand, are more stable than 5 by 20.7 and 8.5 kcal/mol, respectively. The spiro compounds 8, in which the C and B atoms are sandwiched by two allylic μ²-Si₃H₃ ligands, are still more stable by 29.6 and 21.9 kcal/mol, respectively. All three types (face-face, face-side, side-side) of sandwich structures are considered viable targets for synthetic pursuit. The Be complexes deviate from the C and B analogues because Be is much more electropositive. In the preferred cluster structure 9 the Be atom sits in a Si₆H₆ basket.