Cycloaddition reactions of the acyclic silylene Si(SAriPr4)2 (AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2) with a variety of alkenes and alkynes were investigated. Its reactions with the alkynes phenylacetylene and diphenylacetylene and the diene 2,3-dimethyl-1,3-butadiene yielded silacycles (AriPr4S)2tiebar above startSi(CH═tiebar above endCPh) (1), (AriPr4S)2tiebar above startSi(PhC═tiebar above endCPh) (2), and (AriPr4S)2tiebar above startSiCH2CMeCMetiebar above endCH2 (3) at ambient temperature. The compounds were characterized by X-ray crystallography, 1H, 13C, and 29Si NMR spectroscopy, and IR spectroscopy. No reaction was observed with more substituted alkenes such as propene, (Z)-2-butene, tert-butylethylene, cyclopentene, 1-hexene, or the alkyne bis(trimethylsilyl)acetylene under the same reaction conditions. The germylene Ge(SArMe6)2 and stannylene Sn(SArMe6)2 (ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2) analogues of Si(SArMe6)2 displayed no reaction with ethylene. Quantum chemical calculations using model tetrylenes E(SPh)2 (E = Si, Ge, Sn; Ph = C6H5) show that cyclization reactions are endothermic in the case of germanium and tin derivatives but energetically favored for the silicon species.