([eta]5-C5Me5)(CO)2Fe-P=P-Mes* (Mes* = 2,4,6-tBu3C6H2)reacts with one equivalent of the [Delta]1-1,2,4-triazoline-3,5-diones N = N - C(0)- N(R)- C(0) (2a: R = Ph; 2b: R = 4-EtOC6H4) in benzene to give the first 1,2-diaza-3,4-diphosphetidines 3a, b. When the same reaction is performed in diethylether the novel (2E,8E)-1,7-dioxa-3,4,9,lO-tetraaza-5,6,11,12-tetraphospha-2,8-cyclododecadienes 4a, b are isolated as the sole products. The molecular structures of 3a and 4b are established by single-crystal X-ray analysis.