1,1-Diorgano-1-stannacyclopent-3-enes have been synthesized by condensation in THF of magnesium complexes of 1,3-dienes and dichlorodiorganostannanes. 1,1-Dimethyl-, 1,1-di-n-butyl-, 1,1-di-tert-butyl-, and 1,1-diphenyl-1- stannacyclopent-3-enes and 1,1,3,4-tetramethyl-, 1,1-di-tert-butyl-3,4-dimethyl- , and 3,4-dimethyl-1,1-diphenyl-1-stannacyclopent-3-enes were prepared. Kinetic studies of the pyrolysis at temperatures as low as 75 °C of several of these stannacyclopent-3-enes resulted in their first-order disappearance, consistent with a unimolecular dissociation to the corresponding stannylene and diene. Activation parameters are reported. Trapping of dimethylstannylene by dienes was overwhelmed by oligomerization of Me 2Sn:, but for τ-Bu 2Sn: a high yield of diene adduct was obtained. The dimethylstannylene oligomer(s) functioned as stannylenoids and were responsible for several reactions previously attributed to free Me 2Sn:. cydo-(τ-Bu 2Sn)4 may also function as a stannylenoid.