(h5-C5Me5)(CO)2Fe:P-Mes* [Mes* = 2,4,6-(Me3C)3C6H2] reacts with one equiv. of the D-1,2,4-triazoline-3,5-diones I (R = Ph, 4-EtOC6H4) in benzene to give the first 1,2-diaza-3,4-diphosphetidines II. When the same reaction is performed in diethyl ether the novel (2E,8E)-1,7-dioxa-3,4,9,10-tetraaza-5,6,11,12-tetraphospha-2,8-cyclodecadienes III are isolated as the sole products. The mol. structures of II (R = Ph) and III (R = 4-EtOC6H4) are established by single-crystal x-ray anal.