The electron deficient acetylene, hexafluorobut-2-yne, reacts with molybdenum and tungsten methanethiolate derivatives (prepared in situ) to give vinyl and three-, five-, or six-membered heterocyclic derivatives: [Cp(OC)- MoC(O)C(CF 3 )C(CF 3 )C(O)S Me], [Cp(OC)2 MC(CF 3 )C(CF 3 )C(CF 3 )C(O)S Me], [CpW(CO)3C(CF3C(CF3)SMe], [CpW{η3- C(CF 3 )C(CF 3 )C(SMe)OC (O)}-(CO)2]. These reactions contrast with those of trifluoropropyne where no organometallic species are obtained. On heating or irradiation with CF3CCCF3 [CpMH(CO)3] gives known bridged species and in the presence of dimethyl disulphide the vinyl derivative [CpM(CO)3C(CF3)C(CF3)H]and an isomer of undetermined structure.