The η5-thiophene complexes, [(CO)3Mn(η5-T)]+ (1) and [(CO)3Mn(η5-2,5-Me2T)]+ (2), react with LiCuR2 (R = Me or Ph) by adding R- to the sulfur atom of the thiophene, which gives the η4-thiophene complexes, (CO)3Mn(η4-T·R) and (CO)3Mn(η4-2,5-Me2T·R). An X-ray study of (CO)3Mn(η-T·C6H5) (6) shows the η4-T·C6H5 ligand to be coordinated to the manganese through the four thiophene carbon atoms, while the sulfur bearing the phenyl group is bent away from the metal. The structure of 6 supports previous suggestions that the sulfur in η5-thiophene complexes is an electrophilic center. Reactions of 1 or 2 with RS-, CH3O-, or -CH(CO2CH3)2 nucleophiles result in the displacement of thiophene with the formation of [(CO)4Mn(μ-SR)]2 or Mn2(CO)10. The structure of [(CO)4Mn(μ-SC6H4CH3-P)] 2 (9), established by X-ray studies, is also reported.