The first dialkyl-substituted silicon-chalcogen doubly bonded compounds [R2Si=X; R-2=1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl, X=S (4), Se (5), and Te (6)] were synthesized by the reactions of an isolable dialkylsilylene R2Si: ( 3) with phosphine sulfide, elemental selenium, and elemental tellurium, respectively. Systematic changes of characteristics of silicon-chalcogen double bonds are elucidated by X-ray analysis, UV-vis spectroscopy, and DFT calculations. In the solid state, the unsaturated silicon atom in 4-6 adopts planar geometry and the extent of the shortening of Si=X double bonds from the corresponding Si-X single bonds decreases in the order 4 > 5 > 6. In the absorption spectra of 4-6, pi ->pi* transition bands are observed distinctly in addition to n ->pi* transition bands. Both the n ->pi* and pi ->pi* transitions are red-shifted in the order 4 < 5 < 6, and the difference between the energies of the two transitions is kept almost constant among 4-6. The tendency is explained using the qualitative perturbation theory and is reproduced by the DFT calculations for model silanechalcogenones. Addition reactions of water, methanol, and isoprene to 4- 6 are reported.