Comparative chemistry of isolable divalent compounds of silicon, germanium, and tin

Recent studies of the synthesis, structures, spectroscopic properties, and reactions of a series of isolable metallylenes (R ₂E:, E = Si (1), Ge (2), and Sn (3); R ₂ = 1,1,4,4-tetrakis(trimethylsilyl) butane-1,4-diyl) are summarized. Because these group-14 metallylenes bear the same helmet-like ligand, a straightforward discussion of the element-dependence of the intrinsic properties of the group-14 element divalent compounds is possible. All these metallylenes were monomeric both in solution and in the solid state, indicating the effective steric protection by the ligand against dimerization. A small σ-π conjugation between C-Si(substituent) σ orbitals and the vacant np _z orbitals of divalent atoms in R ₂E: exists and the extent decreases in the order E = Si > Ge > Sn, as evidenced by UV-vis and NMR spectroscopies and X-ray crystallography. However, the extent of the σ-π conjugation in metallylenes 1-3 was much smaller than the electron-donating effects of neighboring nitrogen atoms in known stable cyclic diamino-substituted metallylenes, and hence metallylenes 1-3 are regarded as the least electronically perturbed. Comparative studies of the unique reactions among these metallylenes are also discussed.