The cyclic voltammograms of the alkyne complexes [M(SR)L(η-R′CCR′)(η-C₅H₅)] (M = Mo or W, R = Me or Ph, R′ = Me or Ph) show two oxidation processes. Both are irreversible for the stereochemically rigid carbonyls (L = CO) but the first is reversible for the fluxional phosphites {L = P(OMe)₃}; the paramagnetic monocations [M(SPh){P(OMe)₃}(η-MeCCMe)(η-C₅H₅)]⁺ were detected by ESR spectroscopy after in situ chemical one-electron oxidation. By contrast, the hydrotris(pyrazolyl)borate analogues [W(SR)(CO)(η-PhCCPh)Tp′] {R = Me or Ph, Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} are oxidised in two reversible steps to the corresponding mono- and dications; the redox pair [W(SPh)(CO)(η-PhCCPh)Tp′]^z (z = 0 and 1+) has been structurally characterised. A comparison of the redox potentials for the oxidation of [W(SR)(CO)(η-PhCCPh)Tp′] with those of the halide analogues [WX(CO)(η-PhCCPh)Tp′] suggests that the factors which give rise to the inverse halide order for the latter may not operate for the thiolates, which appear to be the better π-donors in all three redox states [WL(CO)(η-PhCCPh)Tp′]^z (L = halide or thiolate, z = 0, 1+ and 2+).