1. ACS - PRF1 MICROWAVE STRUCTURE MEASUREMENTS  Microwave spectroscopy measurements have yielded the first accurate gas- phase structural parameters for Os(CO) 4 (  2 -C 2 H 4 ), tetracarbonylethyleneosmium[i].[i]  The deformation of the ethylene ligand upon coordination to osmium is large, well-determined and supports the DCD model. The new experimental ethylene C—C bond length of 1.43 Å for the complex falls between the free ethylene average r z (C=C) value of 1.339(1) Å and the r 0 (C—C) bond length of 1.534(2) Å for ethane. The C—C bond length for the Fe congener is 1.421(7) Å.  The angle between the plane of the CH 2 group and the C—C bond (  out-of- plane) is 26°. This angle is 22° for Fe(CO) 4 (C 2 H 4 ), indicating that the degree of metal (d  )  olefin (  *) back-bonding is greater for the Os complex.  A homodyne-type detection system, was developed for our pulsed-beam Fourier transform spectrometer system and used for these measurements. This homodyne system greatly simplifies the microwave circuit, with no apparent loss in sensitivity. [i] C. Karunatilaka, B. S. Tackett, J. Washington, and S. G. Kukolich, J. Am. Chem. Soc. 129(34), 10522-10530 (2007) (doi: 10.1021/ja0727969 S0002- 7863(07)02796-5).[i]

2. ACS - PRF2 Ethylene Structure Changes on Complex Formation

3. ACS - PRF3 DCD Model for Alkene-Metal Complexes  Model to describe alkene bonding to a transition metal, proposed by Dewar[1] over 50 years ago.[1]  Model was successfully applied by Chatt and Duncanson[2] for a systematic description of metal-olefin complexes and is now termed the DCD model.[2]  Dewar[3] suggested the existence of two extremes of the metal-olefin bonding, from “simple  -complex” to metallacyclopropane, like classical, strained three- membered ring. Metal-olefin bonds fall somewhere between. ______________________________________________[3] [ 1][ 1] Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71. [2][2] Chatt, J; Duncanson, L. A. J. Chem. Soc. 1953, 2939. [3][3] Dewar, M. J. S.; Ford, G. P. J. Am. Chem. Soc. 1979, 101(4), 783.

4. ACS - PRF4 Homodyne system greatly simplifies the microwave circuit, with no apparent loss in sensitivity[1] Rest of F-T spectrometer is similar to earlier spectrometers.[2][1][2] [1][1] Tackett, B. S.; Karunatilaka, C.; Daly, A.; Kukolich, S. G. Organometallics 2007, 26, 2070-2076 [2][2].Bumgarner, R. E.; Kukolich, S. G. J. Chem. Phys. 1987, 86(3), 1083-1089.

5. ACS - PRF5 Acknowledgements ACS-PRF - Acknowledgement is made to the Donors of The American Chemical Society Petroleum Research Fund, for the partial support of this research. N$F - This material is based upon work supported by the National Science Foundation under Grant No. CHE-0304969. Giles L. Henderson and Richard Keiter of Eastern Illinois University started this spectroscopy project in 1992