The reactivity of diazadiphosphapentalene 1 towards various substrates was investigated. Reaction of 1 with ammonia–borane resulted in transfer hydrogenolysis concomitantly with the cleavage of a P−N bond. By treatment of 1 with 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), oxidation took place at one of the phosphorus atoms of 1, and a PV/PIII mixed‐valence derivative was isolated. At the same time, it was demonstrated that only one of the phosphorus atoms in 1 behaves as an electron donor for electrophiles and Lewis acids. The former afforded an intramolecularly coordinated phosphine‐phosphenium species, whereas the latter demonstrates the ligand property of 1. UV irradiation induced rearrangement of 1 into another example of another diazadiphosphapentalene. Reactivity series: Reactivity of diazadiphosphapentalene 1 towards various substrates was investigated. Activation of ammonia–borane by 1 proceeded concomitantly with the cleavage of the P−N bond in 1, while electrophiles and Lewis acids readily coordinated to the P atom between two carbon atoms in 1 (see scheme). Under irradiation with a Hg(Xe) lamp, photoisomerization of 1 occurred to afford a new diazadiphosphapentalene derivative.