On account of our interest in P₄ activation by phosphenium ion insertion into P–P bonds we have developed synthetic routes to bicyclic N–P–Si-heterocycle 7 and probed its reactivity towards GaCl₃ and P₄. A GaCl₃-induced rearrangement of 7 leads to the in situ formation of spirocyclic, Si-centered phosphenium ions. Their insertion into P–P bonds of one or two P₄ tetrahedra yields polyphosphorus cages [ClP(NSiMe₃)₂Si(NSiMe₃)₂P₅]⁺ (19⁺) and [P₅(NSiMe₃)₂Si(NSiMe₃)₂P₅]²⁺ (13²⁺).