The diphosphenyl complex (h5-C5Me5)(CO)2FeP:PC6H2(CMe3)3-2,4,6 (I) reacts with Me propiolate and Me 2-butynoate in benzene (20 Deg) as well as with di-Me acetylenedicarboxylate (II) in toluene (-30 Deg) to give five-membered metallaheterocycles III [R = H, Me (IV), CO2Me; R1 = OMe] with E exocyclic P:P double bonds. The transformation of I with II and the ketones PhC.tplbond.C(O)Me and PhC.tplbond.C(O)Ph in benzene (0-20 Deg) led to similar heterocycles III (R = CO2Me, R1 = OMe; R = Ph, R1 = Me, Ph) with Z exocyclic P:P double bonds. The novel chelate ligand in III features the structural elements of a 1,2-diphospha-1,3-butadiene. The mol. structure of IV is established by x-ray diffraction.