AbstractThermolysis of [bis(diisopropylamino)phosphanyl](trimethylsilyl)diazomethane1affords the corresponding carbene2, which is stable enough to be spectroscopically characterized. This species possesses a phosphorus–carbon multiple‐bond character as shown by the 2 + 3 and 2 + 2 cycloaddition reactions observed with trimethylsilyl azide or N2O and ethyl cyanoformate, respectively. On the other hand,2undergoes all the classical reactions of a carbene: cyclopropanation reaction with electron‐poor alkenes, carbon–hydrogen bond insertion, and carbene–carbene coupling with isonitriles. Compound2reacts with trimethylsilyl triflate affording a stable methylenephosphonium salt15. Treatment of the lithium salt of the [bis(diisopropylamino)thiophosphoranyl]diazomethane18with the bis(diisopropylamino)phosphanyl chloride leads to a stable nitrilimine3. Thermolysis of3affords the isomeric diazo derivative20, while photolysis gives rise to thiophophoranylnitrile21and cyclodiphosphazene23. Regioselective 2 + 3 cycloadditions are observed with electron‐poor dipolarophiles. Addition of trimethylsilyl triflate to3leads to a stable electrophilic ni