Free stannylenes Me 2Si(tBuN) 2Sn 1a and [(Me 3Si) 2CH] 2Sn 1b as well as SnCl 2, SnBr 2, and SnI 2 give 1,4-cycloadditions to different 1,3-dienes yielding 1-stannacyclopent-3-enes. A singlet heavy carbene analogue behaviour is shown with the two isomers of the diallene 2 : only those stereoisomers are found to be postulated for a [2+4] cheletropic addition. In the other cases, a similar mechanism is likely. 1b acts as a donor towards E,E-1,4-disubstituted 1,3-dienes, reacting faster with electron deficient ones.