Four‐membered rings with a P2BCh core (Ch=S, Se) have been synthesized by the reaction of phosphinidene chalcogenide (Ar*P=Ch) and phosphaborene (Mes*P=BNR2). The mechanistic pathways towards these rings are explained by detailed computational work that confirmed the preference for the formation of P−P, not P−B, bonded systems, which seems counterintuitive given that both phosphorus atoms contain bulky ligands. The reactivity of the newly synthesized heterocycles, as well as that of the known (RPCh)n rings (n=2, 3), was probed by the addition of N‐heterocyclic carbenes, which revealed that all investigated compounds can act as sources of low‐coordinate phosphorus species. Some serious (P2)BS: The combination of a phosphinidene chalcogenide and phosphaborene has created a unique heterocycle containing Group 13, 15, and 16 elements. Detailed theoretical work describes the likely pathway in generating P2BCh heterocycles. The reactivity of these, and other (RPCh)n rings, with N‐heterocyclic carbenes is reported.