The reaction of triethylamine with the [4 + 2] cycloadducts of phosphole sulfides and 3-bromo-N-phenylmaleimide provides a convenient access to phosphinidene sulfides [RP=S]. These transient species are trapped by 2,3-dimethyl-1,3-butadiene to give the previously unknown trivalent [4 + 2] cycloadducts. One of these (R = Ph) has been characterized as its P-W(CO)5 complex by X-ray crystal structure analysis. With cyclopentadiene, the subsequent insertion of a second molecule of [RP=S] leads to a new type of bicyclic product containing a thiadiphospholane ring. © 2014 American Chemical Society.