Ring expansion vs. addition - reactivity of a cyclo -P 4 complex towards pnictogenium cations.

A systematic study on the reactivity of the cyclo -P 4 complexes [Cp R Ta(CO) 2 (η 4 -P 4 )] towards pnictogenium cations results in the formation of functionalised interpnictogen cations. Phosphenium ions insert into one of the P-P bonds to give ring-expanded cyclo -P 5 R 2 products. In contrast, an arsenium-functionalised P 4 AsCy 2 ligand displays an interesting borderline case between ring expansion and coordination, while stibenium cations afford addition products. Tuning of the steric and electronic properties of the stibenium ion shows a drastic influence on the reaction outcome.