STEPHEN KUKOLICH, BRIAN DROUIN, OLIVER INDRIS AND JENNIFER DANNEMILLER, Department of Chemistry, University of Arizona, Tucson, AZ 85721 (Present Address for B. Drouin: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109); JOCHEN ZOLLER AND WOLFGANG HERRMANN, Anorganisch-Chemisches Institut der Technischen Universität München, Lichtenbergstrasse 4, D-85747 Garching bei München, Germany.

\beginwrapfigure[11]r5.5cm \vspace-.5cm \hspace*-1cm \epsfigfile=mry.eps \endwrapfigure The molecular structure for (( eta2-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 of transition 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 exhibits partial sp2 hybridization. The C-C bond length is increased by 0.08Å to 1.29Å. 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.