Synthesis, structures, and reactions of ethenethiolato complexes of molybdenum and tungsten [M{η³-SC(CF₃)=C(CF₃)H}(CF ₃C≡CCF₃)(η⁵-C₅H ₅)](M = Mo or W). Their relevance to the mechanism of metal-promoted isomerisation of carbon-carbon double bonds

Thermolysis of the ?²-C,C-vinyl complexes [M{?³-C(CF₃)C(CF₃)SBu^t}(CF ₃C=CCF₃)(?⁵-C₅H ₅)] (1) (M = Mo or W) gives ?³-ethenethiolato complexes Z-endo-[M{?³-SC(CF₃)=C(CF₃)H}(CF ₃C=CCF₃)(?⁵-C₅H ₅)] (2). With M = W an intermediato E-exo-[W{?³-SC(CF₃)=C(CF₃)H}(CF ₃C=CCF₃)(?⁵-C₅H ₅)] (3) was isolated and structurally characterised, illustrating that exo ? endo and E ? Z isomerisations are concomitant processes. The relevance of this observation for mechanism(s) of metal-pormoted alkyne isomerisations is discussed. Reactions of (2) and (3) with tertiary phosphines and dioxygen give s-ethenethiolato complexes [M{SC(CF₃)=C(CF₃)H}L(CF₃C=CCF ₃)(?⁵-C₅H₅)] (M = Mo, L = O or PPh₃; M = W, L = O, PPh₃, PMe₂Ph, or PPh₂C=CPh) with retention of stereochemistry about the C=C bond. Dynamic ¹⁹F n.m.r. studies reveal fluxional behaviour in some complexes due to alkyne propeller rotation.