3H‐Phosphaallenes, R−P=C=C(H)C−R’ (3), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH3(SMe2) and 3 a (R=Mes*, R’=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane (7) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms. The increased Lewis acidity of the B atom led to a different reaction course upon treatment of 3 a with H2B‐C6F5(SMe2). Hydroboration of a C=C bond of a first phosphaallene is followed in a typical FLP reaction by the coordination of a second phosphaallene molecule via B−C and P−B bond formation to yield a BP2C2 heterocycle (8). Its B−P bond is short and the B‐bound P atom has a planar surrounding. Treatment of 3 a with tBuLi resulted in deprotonation of the β‐C atom of the phosphaallene (9). The Li atom is bound to the P atom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl‐SiMe3 or Cl‐PtBu2. The thermally unstable phosphaallene Ph−P=C=C(H)‐tBu gave a unique trimeric secondary product by P−P, P−C and C−C bond formation. It contains a P2C4 heterocycle and was isolated as a W(CO)4 complex with two P atoms coordinated to W (15). Renewed interest: 3H‐Phosphaallenes are highly functional compounds with a fascinating perspective in various chemical transformations. Hydroboration of the C=C bond afforded a BP2 FLP or a BP2C2 heterocycle and deprotonation yielded a reactive Li phosphide. A new secondary product was obtained by trimerisation of a sterically low shielded phosphaallene via P−P, P−C and C−C bond formation.