Reduction of [ML(alkyne)(2)(eta-C5R '(5))](+) (M = Mo or W, L = MeCN or CO, R ' = H or Me, C5R '(5) = C5HPh4): Characterization of radical intermediates in the reductive coupling of coordinated alkynes

The complexes [M(NCMe)(RC2R)(2)(eta-C5R5')](+) (M = Mo or W, R = Me or Ph; R' = H or Me, C5R5' = C5HPh4) undergo one-electron reduction to [M(NCMe)(RC 2R)(2)(eta-C5R5')], which rapidly lose acetonitrile to give the 17-electron complexes [M(RC2R)(2)(eta-C5R5')]. The stability of [M(RC2R)(2)(eta-C5R5') ] depends on R and R'; for R = Ph, the radical is sufficiently stable so th at its reduction to the 18-electron anion [M(RC2R)(2)(eta-C5R5')](-) is det ected in the cyclic voltammogram. Chemical reduction of [Mo(NCMe)(PhC2Ph)(2 )(eta-C5HPh4)](+) with [Co(eta-C5H5)(2)] gives the air-sensitive solid [Mo( PhC2Ph)(2)(eta-C5HPh4)], characterized as an alkyne-based radical by ESR sp ectroscopy, which gives [Mo(NCMe)(PhC2Ph)(2)(eta-C5HPh4)](+) when treated w ith [Fe(eta-C5H5)(2)](+) in acetonitrile. The carbonyl cations [M(CO)(RC2R) (2)(eta-C5R5')](+) (M = Mo, Tm) undergo two sequential one-electron reducti ons, the first of which is reversible and gives the 19-electron species [M( CO)(RC2R)(2)(eta-C5R5')]. The 19-electron radical [Mo(CO)(PhC2Ph)(2)(eta-C5 Me5)] has been characterized in solution by IR and ESR spectroscopy.