The fac-tricarbonyl manganese(I) species fac-[Mn(CO)₃(L)]⁺ (L = [12]aneS₄, 1,4,7,10-tetrathiacyclododecane; [14]aneS₄, 1,4,8,11-tetrathiacyclotetradecane; or [15]aneS₅, 1,4,7,10,13-pentathiacyclopentadecane), formed by treatment of fac-[Mn(CO)₃(Me₂CO)₃]⁺ with L in MeCN solution, were readily decarbonylated with Me₃NO to yield the first dicarbonyl manganese(I) thioether complexes, cis-[Mn(CO)₂(L)]⁺. The crystal structures of [Mn(CO)₃([12]aneS₄)]CF₃SO₃ and [Mn(CO)₃([15]aneS₅)]CF₃SO₃ confirm the fac tricarbonyl arrangement, with the macrocycle bonded in a tridentate manner, leaving one or two non-bonding S atoms respectively. The crystal structure of cis-[Mn(CO)₂([12]aneS₄)]CF₃SO₃·0.5CHCl₃ reveals a distorted octahedral geometry at Mn^I with the two CO ligands mutually cis and with the tetradentate macrocycle occupying the other four co-ordination sites. IR spectroscopic studies reveal that the CO stretching vibrations occur at very low frequency, indicating very significant π-back bonding to the CO ligands, with the thioether ligands behaving essentially as pure σ donors to the low-valent manganese(I) ion in these systems.