Treatment of [Mo{P(OMe)₃}₂(η²-MeC₂Me)(η-C₅H₅)][BF₄] with NaSR (R = Me, p-NH₂C₆H₄, p-MeOC₆H₄, p-MeC₆H₄, C₆H₅, or p-NO₂C₆H₄) leads to displacement of P(OMe)₃ and formation of [Mo(SR){P(OMe)₃}(η²-MeC₂Me)(η-C₅H₅)]. Similar reactions afforded [Mo(SePh){P(OMe)₃}-(η²-MeC₂Me)(η-C₅H₅)] and [Mo(SMe){P(OMe)₃}(η²-Bu^tC₂H)(η-C₅H₅)]. Barriers to rotation of the but-2-yne ligand present in these complexes are related to electronic effects of the substituents on the thiolate ligand. The molecular structure of [Mo(SC₆H₄NO₂-p){P(OMe)₃}(η²-MeC₂Me)-(η-C₅H₅)] has been established by single-crystal X-ray crystallography. Crystals are monoclinic, space group P2₁/n with a= 8.968(3), b= 20.104(9), c= 11.452(5)Å, β= 95.77(3), and Z= 4. Using 3 601 reflections measured on a four-circle diffractometer at 200 K, the structure has been refined to R 0.028, R′ 0.032. The molecule adopts the familiar three-legged piano stool geometry, in which the but-2-yne ligand is orientated such that the central C–C bond lies approximately parallel to the Mo–P vector. In contrast, [Mo(CO)(η²-MeC₂Me)₂(η-C₅H₅)][BF₄] reacts with NaSMe to give the complexes [Mo(SMe)(CO)(η²-MeC₂Me)(η-C₅H₅)] and [Mo₂(CO)₄(µ-MeC₂Me)(η-C₅H₅)₂], whereas the corresponding reaction with [Mo(NCMe)(η²-MeC₂Me)₂(η-C₅H₅)][BF₄] affords [Mo(SMe)-(η²-MeC₂Me)(η-C₅H₅)]. Reaction of sodium dimethyldithiocarbamate with [Mo(NCMe)(R¹C₂R²)₂-(η-C₅H₅)][BF₄](R¹= R²= Me or R¹= Bu^t, R²= H) gives the monoalkyne complexes [Mo(S₂C–NMe₂)(η²-R¹C₂R²)(η-C₅H₅)]. In contrast the anion [SC₆H₄SPh-o]^– affords a separable mixture of [Mo(SC₆H₄SPh-o)(η²-R¹C₂R²)₂(η-C₅H₅)] and [[graphic omitted]Ph-o)(η²-R¹C₂R²)(η-C₅H₅)]. Reaction of the S,S′-bidentate species with carbon monoxide gives [Mo(SC₆H₄SPh-o)(CO)(η²-MeC₂Me)-(η-C₅H₅)], which was structurally identified by X-ray crystallography. Crystals are orthorhombic, space group P2₁2₁2₁(no. 19) with Z= 4 in a unit cell of dimensions a= 9.852(3), b= 11.486(4), and c= 18.476(6)Å. From 1 714 independent intensities measured at 292 K the structure has been refined to R 0.059, R′ 0.041. The molecule adopts a piano stool geometry with the but-2-yne lying parallel to the Mo–CO vector. The dithioaryl ligand is monodentate.