A new R,ω-bis(phosphine)-polyether ligand, 1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 , has been synthesized and used to prepare a number of conformationally restrained metallacrown ethers. The reaction of 1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 and Mo(CO) 4 (nbd) gives a good yield of cis-Mo-(CO) 4 {1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 -P,P′}. This complex has been isomerized to trans-Mo-(CO) 4 {1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 -P,P′} using both UV light and HgCl 2 catalysis. The reaction of 1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 with fac-RuCl 2 (CO) 3 (THF) initially yields a mixture of monomeric and oligomeric cis,cis,trans-[RuCl 2 (CO) 2 {1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 -P,P′}] n metallacrown ethers. Surprisingly, the oligomeric metallacrown ethers slowly convert into the monomeric metallacrown ether at ambient temperature in a chloroform-d solution. This does not occur with the conformationally mobile [cis,cis,trans-RuCl 2 (CO) 2 {Ph 2 P(CH 2 CH 2 O) 4 -CH 2 CH 2 PPh 2 -P,P′}] n metallacrown ethers. The X-ray crystal structure of cis-Mo(CO) 4 {1,2-(Ph 2 P(CH 2 CH 2 O) 2 ) 2 C 6 H 4 -P,P′} has also been determined. The four ether oxygens lie in a plane, and the average cross-ring oxygen-oxygen distance is slightly larger than those in metallacrown ethers containing Ph 2 P(CH 2 CH 2 O) 4 CH 2 CH 2 PPh 2 ligands.