THE MOLECULAR-STRUCTURE OF (ETA-5-CYCLOPENTADIENYL)BIS(ETHENE)RHODIUM DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION

The molecular structure of [Rh(eta5-C5H5)(C2H4)2] has been determined in the gas phase by electron diffraction at 473 K. The molecule has C2v local symmetry for the Rh(C2H4)2 fragment and C5v for the Rh(C5H5) fragment, with the local rotation axes coincident. The distance from the rhodium atom to the centre of the cyclopentadienyl ring is 190.7(3) pm, and that from the rhodium atom to the centres of the C-C bonds of the ethene ligands is 198.0(3) pm. The C-C bond distances in the cyclopentadienyl ligands refined to 143.2(2) pm, as expected for such five-co-ordinated rings, but the C-C bonds of the ethene ligands [145.7(7) pm] are longer than those previously reported for rhodium-ethene compounds. The angle between the lines linking the midpoints of the C=C bonds of the ethene ligands to the rhodium atom is not uniquely determined by the electron diffraction data, and models with values of 91.2(13) and 111.8(12)-degrees both fit well. The former value is preferred, as it gives a lower R factor, and is consistent with corresponding angles reported for bis(ethene) complexes in the crystalline phase. The two ethene ligands are tilted about their C=C axes away from one another by 11.7(18)-degrees.