The d⁴ halide complexes [MX(CO)(η-RCCR)Tp′] {X = F, Cl, Br or I; R = Me or Ph; M = Mo or W; Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} undergo one-electron oxidation to the d³ monocations [MX(CO)(η-RCCR)Tp′]⁺, isolable for M = W, R = Me. X-Ray structural studies on the redox pairs [WX(CO)(η-MeCCMe)Tp′]^z (X = Cl and Br, z = 0 and 1), the ESR spectra of the cations [WX(CO)(η-RCCR)Tp′]⁺ (X = F, Cl, Br or I; R = Me or Ph), and DFT calculations on [WX(CO)(η-MeCCMe)Tp′]^z (X = F, Cl, Br and I; z = 0 and 1) are consistent with electron removal from a HOMO (of the d⁴ complexes) which is π-antibonding with respect to the W–X bond, π-bonding with respect to the W–C(O) bond, and δ-bonding with respect to the W–C_alkyne bonds. The dependence of both oxidation potential and ν(CO) for [MX(CO)(η-RCCR)Tp′] shows an inverse halide order which is consistent with an ionic component to the M–X bond; the small size of fluorine and its closeness to the metal centre leads to the highest energy HOMO and the lowest oxidation potential. In the cations [MX(CO)(η-RCCR)Tp′]⁺ electronegativity effects become more important, leading to a conventional order for Cl, Br and I. However, high M–F π-donation is still facilitated by the short M–F distance.