MOLECULAR-STRUCTURES OF TRIS(METHYLCYCLOPENTADIENYL)-SCANDIUM AND -YTTERBIUM AS STUDIED BY GAS-PHASE ELECTRON-DIFFRACTION AND MOLECULAR MECHANICS CALCULATIONS - THE SCANDIUM ATOM IS TOO SMALL TO ACCOMMODATE 3 PENTAHAPTO CYCLOPENTADIENYL RINGS

The syntheses and spectroscopic properties (IR and H-1 and C-13 NMR spectral) of tris(methylcyclopentadienyl)-scandium and -ytterbium are described. The vapour pressure of Yb(MeCp)(3) has been determined over the range 70-90 degrees C. The gas phase electron diffraction data for M(MeCp)(3) (M = Sc or Yb) have been recorded with nozzle temperatures of about 160 degrees C. The data for M = Yb, are consistent with a model containing three pentahapto cyclopentadienyl rings; Yb(eta(5)-MeCp)(3), a mean Yb to ring-centre distance of Yb-Z = 236.6(6) pm, and a mean Yb-C bond distance of 265.5(7) pm. The gas phase electron diffraction data for M = Sc are incompatible with models containing three eta(5)-MeCp rings, but consistent with models containing two eta(5) rings and one ring with a hapticity of 2 or 3; eta(2/3)-MeCp. The distance from Sc to the centres of the two eta(5)-MeCp rings is 222.3(6) pm, corresponding to a mean Sc-C(eta(5)) bond distance of 253.0(6) pm. The third ring is at a greater distance from the Sc atom, Sc-Z(eta(2/3))= 252(5) pm, but is tilted, the angle between the Sc-Z(eta(2/3)) vector and the ring normal being 22(4)degrees. As a result two or three carbon atoms of the ring are in a position to form strong bonds to the metal atom. Molecular mechanics calculations indicate that interligand interactions in M(eta(5)-C5H4)(3) or M(eta(5)-MeCp)(3) molecules are strongly repulsive when M-Z(eta(5)) is less than 225 pm, and that the strain is eliminated on rearrangement to (eta(5)-C5H5) (2)(eta(2/3)-C5H5) or M(eta(5)-MeCp)(2)(eta(2/3)(MeCp) configurations.