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 P=C and Si=Si 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 Si=Si 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.