The current study provides a clear understanding of the chemical properties of annelated 3a,6a-diaza-1,4-diphosphapentalenes (DDPs), which are best viewed as stabilized singlet phosphinidenes. It was found that DDPs undergo reversible oligomerization in solution, which provides 1,2,3-diazaphosphole-substituted cyclotetraphosphines, isolated and characterized by X-ray crystal structure analysis. Transformation of the 10-π-electron heteropentalene system into a stabilized phosphinidene occurs when the P-N bond is lengthened, which is facilitated by weak Lewis acids and bases. DFT calculations show that the lowest unoccupied molecular orbital of DDP has a high localization at the phosphorus atom when the N-P bond distance reaches the value of 2.53 Å. Oligomerization is a concentration-dependent process. Increasing the concentration of the monomer solution promotes tetramer formation, and vice versa: a strong dilution leads to a monomer. Tetramer solutions are photosensitive and yield monomers upon irradiation. The new annelated DDP 2 and its dichloro precursor 4 based on tetralone azine were synthesized. 4 exists in the solid state as a 1,4-dichloro isomer, while in solution it gives an equilibrium mixture of 1,1- and 1,4-isomers. Cyclohexanone-annelated diazadiphosphapentalene 1 forms a weak complex (1:1) with Ph3B, showing an elongated P-B bond (2.114(12) Å), which is noticeably larger than the sum of the covalent radii of the elements.