Reduction of [M(CO)₂(η-RCCR′)Tp′]X {Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X = [PF₆]⁻, R = R′ = Ph, C₆H₄OMe-4 or Me; R = Ph, R′ = H; M = W, X = [BF₄]⁻, R = R′ = Ph or Me; R = Ph, R′ = H} with [Co(η-C₅H₅)₂] gave paramagnetic [M(CO)₂(η-RCCR′)Tp′], characterised by IR and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)₂(η-PhCCPh)Tp′] and [Mo(CO)₂(η-PhCCPh)Tp′][PF₆] showed that oxidation is accompanied by a lengthening of the CC bond and shortening of the Mo–C_alkyne bonds, consistent with removal of an electron from an orbital antibonding with respect to the Mo–alkyne bond, and with conversion of the alkyne from a three- to a four-electron donor. Reduction of [Mo(CO)(NCMe)(η-MeCCMe)Tp′][PF₆] with [Co(η-C₅H₅)₂] in CH₂Cl₂ gives [MoCl(CO)(η-MeCCMe)Tp′], via nitrile substitution in [Mo(CO)(NCMe)(η-MeCCMe)Tp′], whereas a similar reaction with [M(CO){P(OCH₂)₃CEt}(η-MeCCMe)Tp′]⁺ (M = Mo or W) gives the phosphite-containing radicals [M(CO){P(OCH₂)₃CEt}(η-MeCCMe)Tp′]. ESR spectroscopic studies and DFT calculations on [M(CO)L(η-MeCCMe)Tp′] {M = Mo or W, L = CO or P(OCH₂)₃CEt} show the SOMO of the neutral d⁵ species (the LUMO of the d⁴ cations) to be largely d_yz in character although much more delocalised in the W complexes. Non-coincidence effects between the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in the SOMO, consistent with a rotation of the coordinated alkyne about the M–C₂ axis.