In situ-generated terminal phosphinidene complex PhPW(CO) 5 adds in a 1,2-fashion to the terminal double bond of tetramethylcumulene and cyclic 1,2,3-cyclodecatriene. The resulting alkenylidenephosphiranes 19A and 20A, which are three-membered phosphiranes containing an exocyclic allenic group, subsequently rearrange to the corresponding phospharadiales 19B and 20B, which are phosphiranes having two exocyclic double bonds. All four organophosphorus compounds were characterized by single-crystal X-ray structure determinations. Bicyclic 20A contains a significantly bent (171.5(7)°) and twisted (14.2(8)°) allenic unit in contrast to 19A. The rearrangement to the thermodynamically favored radialenes is considered to occur by a phosphirane ring opening/closure sequence. On using a second equivalent of PhPW(CO)5, insertion takes place into a PC bond of 20B, but not of 19B, to give two new phospharadialene isomers, viz. cis-20C and trans-20C, both of which were characterized by crystal structure determinations. The PPCC ring in these systems is significantly puckered (∼150°), causing the olefinic bonds of the butadiene unit to be much twisted from planarity. Both phospharadialenes undergo Diels - Alder reactions with methyl-1,2,4-triazole-3,5-dione (MTAD), resulting in the case of the acyclic cumulene in the expected addition product 19D of which the phosphirane ring easily hydrolyzes. Cycloaddition of MTAD to 20B does not occur at the radialene's diene unit but rather invokes one of its PC bonds, possibly in a concerted [(σ2 + π2) + π2] mechanism, to give the unexpected adduct 20F. Ab initio theoretical studies on the parent systems, using the G3(MP2) method, show phospharadialene to be 3.6 kcal/mol more stable than ethenylidenephosphirane. Their strain energies (SE) are calculated to be 32.3 and 29.7 kcal/mol, respectively. The 22.2 kcal/mol SE of phosphirane increases by 5.9 kcal/mol on introducing one exocyclic double bond and by another 4.2 kcal/mol on introducing the second one. Still, the SE of phospharadialene is less than the 39.0 kcal/mol of the more condensed-phosphirene.