In quest of molybdenum complexes that have sulfur donors in their coordination sphere and exhibit reactivity toward small molecules such as CO, NO, NO+, N-3(-), and above all N-2, the chemistry of [Mo("(R)XS(4)")] complexes has been investigated. Mo(II) complexes of the type [Mo(CO)(2)("(R)XS(4)")] (R = H, X = S, 1a; R = tert-butyl, X = S, 1b; R = H, X = NH, 2; R = H, X = 0, 3) formed by reaction of [Mo(CO)(4)(Cl)(2)] or [Mo(CO)(3)(PPh(3))(2)(Cl)(2)] and the thiols "(R)XS(4)"-H-2 or corresponding thiolates. Complex la served as a structural reference and proved the Mo center to be seven-coordinate. The [Mo("S-5")] fragment approximately forms a square pyramid at the base of which the two cis CO Ligands are coordinated. Crystal data of 1a 0.5Et(2)O : monoclinic space group P2(1)/c; a = 14.182(7) Angstrom, b = 14.525(11) Angstrom, c = 11.421(15) Angstrom beta = 108.98(7)degrees, Z = 4, R/R(w) = 6.0/5.6. The [Mo("S-5")] fragment of 1a exhibits the meso-configuration, and NMR spectra indicate the same configuration for the [Mo("(R)XS(4)")] fragments of the other CO complexes. Oxidation of 1-3 yielded the C-s symmetrical Mo(IV) oxo complexes [Mo(=O)("(R)XS(4)")] (R = H, X = S, 4a; R = tert-butyl, X = S, 4b; R = H, X = NH, 5; R = H, X = 0, 6). : monoclinic space group P2(1)/c; a = 14.182(7) Angstrom, b = 14.525(11) Angstrom, c = 11.421(15) Angstrom beta = 108.98(7)degrees, Z = 4, R/R(w) = 6.0/5.6. The [Mo("S-5")] fragment of 1a exhibits the meso-configuration, and NMR spectra indicate the same configuration for the [Mo("(R)XS(4)")] fragments of the other CO complexes. Oxidation of 1-3 yielded the C-s symmetrical Mo(IV) oxo complexes [Mo(=O)("(R)XS(4)")] (R = H, X = S, 4a; R = tert-butyl, X = S, 4b; R = H, X = NH, 5; R = H, X = 0, 6). : monoclinic space group P2(1)/c; a = 14.182(7) Angstrom, b = 14.525(11) Angstrom, c = 11.421(15) Angstrom beta = 108.98(7)degrees, Z = 4, R/R(w) = 6.0/5.6. The [Mo("S-5")] fragment of 1a exhibits the meso-configuration, and NMR spectra indicate the same configuration for the [Mo("(R)XS(4)")] fragments of the other CO complexes. Oxidation of 1-3 yielded the C-s symmetrical Mo(IV) oxo complexes [Mo(=O)("(R)XS(4)")] (R = H, X = S, 4a; R = tert-butyl, X = S, 4b; R = H, X = NH, 5; R = H, X = 0, 6).