C-[Bis(diisopropylamino)thioxophosphoranyl]-N-[bis(diisopropylamino)phosphino] nitrilimine(1) reacts at -50 degrees C with tetrachloro-o-benzoquinone (TCBQ) leading to C-thioxophosphoranyl-N-phosphoranyldiazirine 3, which rearranges above -30 degrees C into heterocycle 4 (84% yield). 1H-Diazirine 3, which has been characterized by P-31 and C-13 NMR spectroscopy, reacts with triisopropylsilyl trifluoromethanesulfonate and trimethylphosphine, affording N-phosphonionitrilimine 5 and phosphorus ylide 6 in 51 and 95% yields, respectively. [Bis(diisopropylamino)thioxophosphoranyl][bis(diisopropylamino)phosphino]diazomethane (8) also reacts with TCBQ leading to 4 (90% yield) via 3. This result is rationalized by the formation of the (thioxophosphoranyl)(phosphoranyl)diazomethane 9, which is in equilibrium with the ion pair thioxophoranyl diazo anion 21/phosphonium 12; attack of the nitrogen end of the diazo anion on the phosphorus cation 12 affords 3. The transient formation of the ion pair 21/12 has been proved by trapping reactions with CH3OD and CDCl3 and by the reaction of the lithium salt of the [bis(diisopropylamino)thioxophosphoranyl] diazomethane (13) with phosphonium salt 12, which also leads to 3 and 4 (82% yield).