Synthesis and reactions of the co-ordinatively unsaturated hexafluorobut-2-yne complex [WBr₂(CO)(CF₃C≡CCF₃)₂]. the crystal and molecular structure of [{W[C(CF₃)=C(CF₃)P(OEt)₂O](μ-Br)(CO)(CF ₃C≡CCF₃)}₂]

Reaction of [{WBr₂(CO)₄}₂] with CF₃C=CCF₃ in pentane affords the bis(alkyne) complex [WBr₂(CO)(CF₃C=CCF₃)₂] (1) in which the carbonyl ligand appears to function as a s-donor ligand only, according to i.r. data. Complex (1) reacts with P(OMe)₃ and P(OEt)₃ to give dimeric complexes [{W[C(CF₃)=C(CF₃)P(OR)₂O](µ-Br)(CO)(CF ₃C=CCF₃)}₂] containing a novel vinylphosphonate ligand resulting from a Michaelis-Arbuzov type elimination of alkyl bromide. The structure of the complex with R = Et has been determined by X-ray analysis. Four centrosymmetric molecules occupy a monoclinic cell with a = 24.068(2), b = 8.440(2), c = 25.197(2) Å, ß = 121.50(1)°, space group I2/a (no. 15, C_2h⁶), and R = 0.036 for 2 872 reflections. The tungsten co-ordination is octahedral with mutually cis carbonyl and s-alkenyl carbon atoms both trans to µ-Br groups, and with phosphate O trans to a four-electron donor ?²-CF₃C=CCF₃. The bonding in the novel vinylphosphonate W[C(CF₃)=C(CF₃)P(OEt)₂O] chelate ring is discussed.