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The d4halide complexes [MX(CO)(η-RCCR)Tp′] {R = Me, M = W, X = F; R = Ph, M = Mo or W, X = F or Cl; Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} undergo two-electron oxidation in the presence of a halide source to give the d2monocations [MX1X2(η-PhCCPh)Tp′](R = Me, M = W, X1= X2= F; R = Ph, M = Mo, X1= X2= F or Cl; M = W, X1= X2= F or Cl; X1= F, X2= Cl). Each monocation (R = Ph) shows two reversible one-electron reductions (the second process was not detected for R = Me) corresponding to the stepwise formation of the neutral d3and monoanionic d4analogues, [MX1X2(η-PhCCPh)Tp′] and [MX1X2(η-PhCCPh)Tp′]−respectively; the potentials for the two processes can be ‘tuned’ over a range of ca.1.0 V by varying M and X. Chemical one-electron reduction of [MX2(η-PhCCPh)Tp′]膫 [MX2(η-PhCCPh)Tp′] (M = Mo or W, X = F or Cl). X-Ray structural studies on the redox pairs [WX2(η-PhCCPh)Tp′]z(X = F and Cl, z= 0 and 1) show the alkyne to bisect the X–W–X angle in the d2cations but align more closely with one M–X bond in the neutral d3molecules, consistent with the anisotropic ESR spectra of the latter; the solution ESR spectrum of [MoF2(η-PhCCPh)Tp′] showed equivalent fluorine atoms, i.ethe alkyne oscillates at room temperature. The successful isolation of [MX2(η-PhCCPh)Tp′]橷 [MX2(η-PhCCPh)Tp′] completes a series in which d6to d2alkyne complexes are linked in a redox family tree by sequential one-electron transfer and substitution reactions. The implications for such trees in the production of new species and the selective synthesis of paramagnetic complexes acting as synthetically useful ‘alkyne radicals’ are discussed. [ABSTRACT FROM AUTHOR]