Bridging alkenyl species from the protonation of ditungsten, tungsten-molybdenum and dimolybdenum transverse alkyne complexes with HCl and CF3COOH. Crystal structure of [WMoCl{μ-(E)-HCCHPh}(CO)4(η5-C5H5)2]

Some Group 6 transition metal complexes with bridging alkyne ligands [MM′(μ-HCCMe)(CO)4(η5-C5H5)2] [MM′ = W2 1; WMo 2; Mo2 3] and [WMo(μ-HCCPh)(CO)4(η5-C5H5)2] (5) were treated with hydrogen chloride to afford respectively [MM′Cl(μ-HCCHMe)(CO)4(η5-C5H5)2] [MM′=W2 (6); WMo (7a); Mo2 (8)] and [WMoCl(μ-HCCHPh)(CO)4(η5-C5H5)2] (9a). Similarly, the complexes [WMo(μ-HCCMe)(CO)4(η5-C5H5)2] (2) and [MM′(μ-HCCPh)(CO)4(η5-C5H5)2] [MM′=W2 (4); WMo (5)] were treated with trifluoroacetic acid to give respectively [WMo(CF3COO)(μ-HC=CHMe)(CO)4(η5-C5H5)2] (7b) and [MM′(CF3COO)(μ-HC=CHPh)(CO)4(η5-C5H5)2] [MM′ = W2 (10)]. All the products 6–10 result from anti Markovnikov addition of a proton to the bridging alkyne groups in 1–5, and contain (E)-alkenyl ligands, μ-HC=CHR, σ-bonded to one metal centre and π-bonded to the other. The metal centre to which the alkenyl ligand is σ-bonded in 6–10 is also coordinated by a conjugate base (Cl− or CF3COO−). In the case of 9a a single-crystal X-ray diffraction study shows that the W centre has been formally oxidised from W(+1) to W(+3), with both the alkenyl and chloro groups σ-bonded to it, while the Mo centre is π-coordinated by the alkenyl group and remains in oxidation state (+1).