Reaction of [M(CO)4(η2-C2H2)] (M=Fe, Os) with [(η5-C5H5)(CO)2WCC6H5]; unexpected substitution of acetylene, formation and molecular structure of [MW(μ-CC6H5)(CO)6(η5-C5H5)]

Reaction of [M(CO)4(η2-C2H2)] (M=Fe, Os) with [(η5-C5H5)(CO)2WCC6H5] affords the heterobimetallic complexes [MW(μ-CC6H5)(CO)6(η5-C5H5)] (M=Fe, 1; Os, 2). This is contrary to the usual substitution reactions of the former compounds, which proceed by loss of CO and not that of the acetylene ligand. The IR and NMR signatures of the compounds are in accord with their formulation and this has been corroborated by single crystal X-ray crystallography. Each compound contains a d8 M(CO)4 fragment. The geometry of the Group 8 metal is trigonal bipyramidal (tbp) with a tungsten-carbyne unit, acting as a two-electron pseudo-alkyne ligand, occupying an in-plane equatorial position. In solution the compounds are fluxional. At room temperature, local scrambling at iron averages all four terminal carbonyl ligands, whereas the process is slower for osmium and results only in broadened axial and equatorial CO resonances in the 13C-NMR spectrum. Intermetallic CO exchange is not observed on the NMR time scale. Plausible scenarios for the formation of 1 and 2 are outlined.