Metallacrown ethers of the type [PdX {Ph P-(CH CH O) CH CH PPh -P,P'}](m) (X = Cl, I) exhibit a unique and reversible mode of molecular recognition in which PdX units are incorporated into the metallacrown ether ring via a ring-expansion reaction to form dimetallacrown ethers. 2 2 2 2 4 2 2 2 2
