Ethylene's cycloadditions to unsaturated hydrocarbons occupy well-established ground in classical organic chemistry. In contrast, its reactivity toward alkene and alkyne analogs of carbon's heavier-element congeners silicon, germanium, tin, or lead has been little explored. We show here that treatment of the distannynes Ar(iPr4)SnSnAr(iPr4) [Ar(iPr4) = C6H3-2,6(C6H3-2,6-iPr2)2, 1] or Ar(iPr8)SnSnAr(iPr8) [Ar(iPr8) = C6H-2,6(C6H2-2,4,6-iPr3)2-3,5-iPr2, 2] with ethylene under ambient conditions affords the cycloadducts Ar(iPr4) Sn(mu2:nu1:n1-C2H4)2Sn Ar(iPr4 (3) or Ar(iPrs) Sn(mu2:nu1:nu1-C2H4)2Sn AriPrs (4) that were structurally and spectroscopically characterized. Ethylene incorporation in 3 and 4 involves tin-carbon sigma bonding and is shown to be fully reversible under ambient conditions; hydrocarbon solutions of 3 or 4 revert to the distannynes 1 or 2 with ethylene elimination under reduced pressure or upon standing at approximately 25 degrees C. Variable-temperature proton nuclear magnetic resonance studies showed that the enthalpies of reaction were near -48 (3) and -27 (4) kilojoules per mole.