The reactivity of 4-membered (RPCh)2 rings (Ch = S, Se) that contain phosphorus in the +3 oxidation state is reported. These compounds undergo ring expansion to (RPCh)3 with the addition of a Lewis base. The 6-membered rings were found to be more stable than the 4membered precursors, and the mechanism of their formation was investigated experimentally and by density functional theory calculations. The computational work identified two plausible mechanisms involving a phosphinidene chalcogenide intermediate, either as a free species or stabilized by a suitable base. Both the 4-and 6-membered rings were found to react with coinage metals, giving the same products: (RPCh)3 rings bound to the metal centre from the phosphorus atom in tripodal fashion. ASSOCIATED CONTENT Computational details, complete experimental section and associated spectroscopic data not included in the manuscript can be found in the supporting information. The crystallographic data for the compounds reported can be found using CCDC numbers 1569577-1569582, all of which are available free of charge.