Syntheses and reactions of η²-vinyl complexes of molybdenum and tungsten [M(SC₆F₅){η²-C(CF₃)C(CF ₃)(PR₃)}-(CF₃C≡CCF₃)(cp)] and [mo{η³-C(CF₃)C(CF₃)(SPr ⁱ)}-(CF₃C≡CCF₃)(cp)] (cp = η-C₅H₅)

Reactions of bis(hexafluorobut-2-yne) complexes [M(SC₆F₅)(CF₃C=CCF₃) ₂(cp)] (M = Mo, 1a; or W, 1b with tertiary phosphines and phosphites (L) give ?²-vinyl complexes [M(SC₆F₅){?²-C(CF₃)C(CF ₃)L}-(CF₃C=CCF₃)(cp)] 3 [(M = Mo or W; L = PEt₃, PMe₂Ph or PMePh₂; M = W, L = P(OMe)₃] as a result of attack at an alkyne carbon. Several isomeric forms of complexes 3 have been detected by NMR spectroscopy. Reactions of phosphines with the ?²-C,C vinyl complex [Mo{?³-C(CF₃)C(CF₃)SPrⁱ}(CF ₃C=CCF₃)(cp)] 2 in contrast give ?⁴-butadienyl derivatives [Mo{C(CF₃)=C(CF₃)C(CF₃)=C(CF ₃)-(SPrⁱ)}L(cp)] 4 (L = PEt₃, PMe₂Ph, or MeC=CPPH₂) apparently as a result of insertion of the alkyne into the M=C bond of the ?²-C,C vinyl. A similar complex 4 is obtained when 3 (M = Mo, L = PMe₂Ph) is allowed to isomerise in diethyl ether at room temperature. In contrast 3 (M = Mo, L = PMe₂Ph) isomerises in refluxing hexane to the tetrakis(trifluoromethyl)cyclobutadiene complex [Mo(SC₆F₅)-(PMe₂Ph){?⁴-C ₄(CF₃)₄}(cp)] 9. An alternative isomerisation process is observed in diethyl ether at 18°C where the product [MoF{?³-C(CF₂)C(CF₃)=C(CF ₃)C(CF₃)(SC₆F₅)}L(cp)] 8 (L = PEt₃ or PMe₂Ph) results from linking of two CF₃C=CCF₃ ligands and a C₆F₅S group and fluorine transfer from a CF₃ to the metal. Fluxional behaviour in complexes 3,8 and 9 has been studied by ¹⁹F NMR spectroscopy.