Room temperature reaction of the uranium(IV)–carbene [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(μ-Cl)Li(TMEDA)(μ-TMEDA)_0.5]₂ (1, BIPM^TMS = C(PPh₂NSiMe₃)₂) with white phosphorus (P₄) produces the organo-P₅ compound [P₅{C(SiMe₃)(PPh₂)}₂][Li(TMEDA)₂] (2) and the uranium(IV)–methanediide [U{BIPM^TMS}{Cl}{μ-Cl}₂{Li(TMEDA)}] (3). This is an unprecedented example of cooperative metal–carbene P₄ activation/insertion into a metal–carbon double bond and also an actinide complex reacting with P₄ to directly form an organophosphorus species. Conducting the reaction at low temperature permits the isolation of the diuranium(IV) complex [{U(BIPM^TMS)([μ-η²:η²-P₂]C[SiMe₃][PPh₂])}₂] (4), which then converts to 2 and 3. Thus, surprisingly, in contrast to all other actinide P₄ reactivity, although this reaction produces catenation overall it proceeds via P₄ cleavage to functionalised P₂ units. Hence, this work establishes a proof of concept synthetic cycle for direct fragmentation, catenation, and functionalisation of P₄.