The reactivity of the 2-phosphaethynolate anion (PCO(-)) towards a cyclic trisilene (cSi3(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)Si3(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.
