Although BH₃ simply coordinates the endocyclic P of (phospholidino)phosphaketene 1^Dipp, the bulkier B(C₆F₅)₃ gives rise to a zwitterionic diphosphirenium, which is a novel type of 2π-electron aromatic system as shown by the calculated NICS values. While the reaction of 1^Dipp with Na[PCO(dioxane)_x] is unselective, the same reaction with the sterically bulky (phospholidino)phosphaketene 1^Ar** [Ar** = 2,6-bis[di(4-tert-butylphenyl)methyl]-4-methylphenyl selectively affords a sodium bridged dimer containing a hitherto unknown λ³,λ⁵,λ³-triphosphete core. The latter formally results from “P⁻” addition to a 1,3-P/C-dipole. Similarly, adamantyl isonitrile adds to 1^Dipp giving a 4-membered phosphacycle. In contrast to 1, the phosphaketene derived from the electrophilic diazaphospholidine-4,5-dione is unstable and reacts with a second molecule of Na[PCO(dioxane)_x] to afford a 1,3,4-oxadiphospholonide derivative.