Knowledge Bank

The molecular structure for ($\eta 2$-acetylene) methyldioxorhenium (ACMDO) was obtained by measuring and analysing the rotational spectra for 14 isotopomers. This appears to be the first gas-phase measurement for a rhenium metalacyclopropene, and the first structural data on this compound. This complex is closely related to reaction intermediates in methyltrioxorhenium (MTO) and $OsO4$ catalysed oxidation reactions. These reactions are very important in industrial chemical production and for syntheses of chiral products. Only a few alkyne complexes or transitions metals in high oxidation states are known and even fewer have been structurally characterized. The microwave spectra were measured in the 4-11 GHz range using a Flygare-Balle-type pulsed-beam microwave spectrometer. Rotational constants and rhenium quadrupole coupling tensors were obtained for all isotopomers. The acetylene ligand structure is modified and exhibis partial $sp2$ hybridization. The C-C bond length is increased by 0.08{\AA} to 1.29{\AA}. The H-C-C interbond angles are reduced from $180\circ$ to $141\circ$, and $152\circ$. The quadrupole splitting patterns and systematic changes in the quadrupole coupling tensors were very helpful in making positive assignments.