Reactions between Group VIB metal hexacarbonyl M(CO) 6 (M Cr, Mo, W) and N,N′-dimethylimidazolidine-2-thione (T), either at elevated temperature or under irradiation by ultraviolet light, produced reactive, moderately stable pentacarbonyl thione complexes, M(CO) 5T. The physical and chemical properties of these complexes are similar, indicating that their structures are the same. The IR bands due to the carbonyls in CHCl 3 solution consist of four peaks at ca. 2060, 1970, 1920 and 1870 cm −1, but five bands were observed for their solids with an additional band at ca. 1940 cm −1, possibly due to restricted rotation of the thione in the solid which reduces their pseudosymmetry. These thione complexes reacted with tetraethylthiuramdisulfide, Et 4tds, at room temperature, as opposed to higher temperature required for the reaction between metal hexacarbonyls and Et 4tds. With equal molar quantities of M(CO) 5T and Et 4tds, complexes M(CO) 4(Et 4tds) are formed, but with excess Et 4tds the thiuramdisulfide is reduced to dithiocarbamate (Et 2dtc) resulting in the formation of Cr(Et 2dtc) 3 and M(Et 2dtc) 4 (M Mo or W) as the final products, although M(CO) 4(Et 4tds) might have been an intermediate.