The metallophosphaalkene (h5-C5Me5)(CO)2FeP:C(NMe2)2 (I) was converted into the stable metalated 1-phospha-1,3-butadiene, (h5-C5Me5)(CO)2FeP:C(E)C(E):C(NMe2)2 (II) and the [3+2]-cycloadduct III (R = R1 = CO2Me) by treatment with EC.tplbond.CE (E = CO2Me). A similar reaction of I with MeC.tplbond.CE (E = CO2Me) resulted in the formation of the 1,2-dihydrophosphete IV. The cycloadduct III (R = Ph, R1 = Me3CO) was the sole tractable product from the reaction of I with the alkyne PhC.tplbond.CC(O)CMe3. The compds. were characterized by elemental anal. as well as by spectroscopic data (IR, 1H, 13C, 31P NMR, MS). The mol. structures of II, III (R = R1 = CO2Me), and IV were detd. by single crystal x-ray anal.