| Abstract
| - The first dialkyl-substituted silicon−chalcogen doubly bonded compounds [R2SiX; R2 = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl, X = S (4), Se (5), and Te (6) ] were synthesized by the reactions ofan isolable dialkylsilylene R2Si: (3) with phosphine sulfide, elemental selenium, and elemental tellurium,respectively. Systematic changes of characteristics of silicon−chalcogen double bonds are elucidated byX-ray analysis, UV−vis spectroscopy, and DFT calculations. In the solid state, the unsaturated silicon atomin 4−6 adopts planar geometry and the extent of the shortening of SiX double bonds from thecorresponding Si−X single bonds decreases in the order 4> 5> 6. In the absorption spectra of 4−6, π→ π* transition bands are observed distinctly in addition to n → π* transition bands. Both the n → π* andπ → π* transitions are red-shifted in the order 4< 5< 6, and the difference between the energies of thetwo transitions is kept almost constant among 4−6. The tendency is explained using the qualitativeperturbation theory and is reproduced by the DFT calculations for model silanechalcogenones. Additionreactions of water, methanol, and isoprene to 4−6 are reported.
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