Carbon dioxide and two equivalents of Na(OCP) form, in an equilibrium reaction, a CO₂ adduct of the composition Na₂(P₂C₃O₄). The anion of this salt, [O₂C–P(CO)₂P]²⁻, is built up by a four-membered 1,3-diphosphetane-2,4-dione ring and a carboxylate unit attached to one of the phosphorus atoms. A remarkable π-delocalization was observed within the OCPCO moiety. The stepwise reaction mechanism leading to Na₂(P₂C₃O₄) was investigated with quantum chemical calculations. Accompanied by the release of CO₂, Na₂(P₂C₃O₄) reacts with both 2-iodopropane and 4,4′,4′′-trimethoxytriphenylmethyl chloride to form four-membered cyclic anions. For comparison the analogous reactions were performed with Na(OCP) instead of Na₂(P₂C₃O₄) and the results are discussed in detail.

The reactivity of Na(OCP) was investigated towards triorganyl compounds of the heavier group 14 elements (R₃EX R = Ph or ⁱPr; E = Si, Ge, Sn, Pb; X = Cl, OTf). In the case of E = Si two constitutional isomers were formed and characterised in situ: R₃Si–O–CP is the kinetic and R₃Si–PCO is the thermodynamic product, representing experimental evidence of the ambident character of the (OCP)⁻ anion. Applying theoretical calculations and spectroscopic methods, the compound previously reported as ⁱPr₃Si–O–CP can now unambiguously be identified as ⁱPr₃Si–PCO. The heavier analogues form exclusively the phosphaketene isomer R₃E–PCO (E = Ge, Sn, Pb). DFT calculations were performed to gain deeper insight into the bonding and thermodynamic stability of these compounds.

We show that the 2-phosphaethynolate anion, OCP⁻, is a simple and efficient catalyst for the cyclotrimerization of isocyanates. This process proceeds step-wise and involves five-membered heterocycles, namely 1,4,2-diazaphospholidine-3,5-dionide anions and spiro-phosphoranides as detectable intermediates, both of which were also found to be involved in the catalytic conversion. These species can be considered as adducts of a phosphide anion with two and four isocyanate molecules, respectively, demonstrating that the OCP⁻ anion acts as a formal “P⁻” source. The interconversion between these anionic species was found to be reversible, allowing them to serve as reservoirs for unique phosphorus-based living-catalysts for isocyanate trimerization.