Phosphide Delivery to a Cyclotrisilene

The reactivity of the 2‐phosphaethynolate anion (PCO − ) towards a cyclic trisilene (cSi 3 (Tip) 4 ) is reported. The result is the net activation of the PC and SiSi multiple bonds of the precursors affording a heteroatomic bicyclo[1.1.1]pentan‐2‐one analogue ([P(CO)Si 3 (Tip) 4 ] − ; 1 ). This reaction can be interpreted as the formal addition of a phosphide and a carbonyl across the SiSi double bond. Photolytic decarbonylation of 1 results in the incorporation of the phosphide vertex into the cyclotrisilene scaffold, yielding a congener of the cyclobutene anion with considerable allylic character.