Metallacrown ethers are a unique class of complexes that form when R2PX-(CH2CH20)nCH2CH2XPR2(X - - 0; R = Ph, alkoxy; n = 3–5) ligands chelate to transition metals. Metallacrown ethers with cis-coordinated phosphines (cw-met-allacrown ethers) contain a crown ether-like pocket and exhibit size-selective binding of alkali metal cations similar to that observed with the crown ethers. Binding of Li+and Hg2+to m-MoCO) (R2PX(CH2CH20)nCH2CH2XPPh2-P, P metallacrown ethers can, under certain circumstances, activate the carbonyl ligands cis to the phosphines to nucleophilic attack or dissociation. The cis-metallacrown ethers are also capable of binding water via coordination to the metal center and hydrogen bonding to the ethers. Recently, octahedral metallacrown ethers with trans-coordi-nated phosphines (frows-metallacrown ethers) have been prepared. The conformations of these complexes are quite different from those of the czj-metallacrown ethers with more extended metallacrown ether chains and the ether oxygens closer to the ligands on the octahedral metal center. The metal centers in the trans-metallacrown ethers appear to spin freely within the metallacmwn ether cavity, indicating that the barriers to rotation about the P-M-P axes are low. Dimetallacrown ethers in which the R2PX(CH2CH20)nCH2CH2XPR2ligands bridge two metal centers have also been isolated. These complexes contain two separate crown ether sites and may be able to coordinate two metal cations. The above results suggest that metallacrown ethers containing platinum-group metal centers could exhibit quite different catalytic activities and selectivities from those of related complexes with simple phosphorus-donor ligands. © 1995, Taylor & Francis Group, LLC. All rights reserved.