Co-ordinatively unsaturated alkyne complexes of molybdenum and tungsten: Reactions and dynamic nuclear magnetic resonance studies of [m(SC₆F₅)(CO)(CF₃C≡CCF ₃)(η⁵-C₅H₅)] (M = Mo or W) complexes

Variable-temperature ¹⁹F n.m.r. studies of the hexafluorobut-2-yne complex [W(SC₆F₅)(CO)(CF₃C=CCF ₃)(?⁵-C₅H₅)] reveal three distinct phases of fluxional behaviour between -90 and +80°C, attributed to restricted rotation about (a) the S-C₆F₅ bond, (b) the W-SC₆F₅ bond, and (c) the CF₃CCCF₃-W bond ('propeller rotation'), in increasing order of energy. The oxo complex [W(SC₆F₅)(O)(CF₃C=CCF ₃)(?⁵-C₅H₅)], obtained by air oxidation of the parent carbonyl in diethyl ether, in contrast shows only restricted rotation about the W-SC₆F₅ bond over the same temperature range. Reactions of the carbonyls [M(SC₆F₅)(CO)(CF₃C=CCF ₃)(?⁵-C₅H₅)] (M = Mo or W) with tertiary phosphines and phosphites (L) give carbonyl-substitution products [M(SC₆F₅)(L)(CF₃C=CCF ₃)(?⁵-C₅H₅)] [M = Mo or W, L = P(OMe)₃, PMe₂Ph, or PEt₃; M = Mo, L = PPh₃ or PMePh₂] under unusually mild conditions which undergo alkyne propeller rotation according to ¹⁹F n.m.r. studies. In the case of alkylphosphines, carbonyl substitution was observed to proceed via thermally unstable intermediates, [M(SC₆F₅)(CO){?²-C(CF₃)C(CF ₃)L}(?⁵-C₅H₅)], isolated and characterised for the case M = W, L = PEt₃. N.m.r. data indicate the presence of two isomeric forms containing an ?²-vinyl ligand C(CF₃)C(CF₃)L resulting from attack of the phosphine at an alkyne carbon atom.