Photolysis of the cis-Mo(CO) 4 {Ph 2 P(CH 2 CH 2 O) n CH 2 CH 2 PPh 2 -P,P%}, (1, n= 3; 2, n= 4; 3, n= 5) metallacrown ethers in tetrahydrofuran under nitrogen gives moderate yields of the corresponding trans-Mo(CO) 4 {Ph 2 P(CH 2 CH 2 O) n CH 2 CH 2 PPh 2 -P,P%} (4, n =3; 5, n= 4; 6, n= 5) metallacrown ethers. The trans-metallacrown ethers are also obtained when catalytic amounts of HgCl 2 are added to chloroform solutions of the cis-metallacrown ethers. The 1 ? 4 and 2 ? 5 equilibria are established within 3 min at ambient temperature and approximately equal amounts of the cis-and trans-metallacrown ethers are present in the equilibrium mixtures. The 3 ? 6 equilibrium is more complicated because HgCl 2 complexation by 3 also occurs in these solutions. Addition of excess HgCl 2 to the equilibrium mixture of 3 and 6 results in the formation of the previously reported cis-Mo(CO) 4 {m-Ph 2 P(CH 2 CH 2 O) 5 CH 2 CH 2 PPh 2 -P,P%,O,O%,O¦,O §,O¨}HgCl 2 . Comparison of the X-ray crystal structures of the trans-metallacrown ethers 4 -6 and trans-Mo(CO) 4 {Ph 2 P(CH 2 CH 2 O) 2 -1-C 6 H 4 -2-(OCH 2 CH 2 ) 2 PPh 2 -P,P%} (7) indicates that both ring size and flexibility affect the conformations of the metallacrown ether rings. The larger oxygen-oxygen distances in the trans-metallacrown ethers, compared with those in the corresponding cis-metallacrown ethers 1, 3 and cis-Mo(CO) 4 {Ph 2 P(CH 2 CH 2 O) 2 -1-C 6 H 4 -2-(OCH 2 CH 2 ) 2 PPh 2 -P,P%} (8), may explain why the trans-metallacrown ethers have not been observed to bind strongly the hard metal cations.