The equilibrium structures of monomeric group 2 and lanthanides(II) metallocenes MCp{sub 2} (M = Ca, Sr, Ba, Sm, Eu, Yb) studied by ab initio calculations
- Universitaet Erlangen-Nuernberg (Germany); and others
The equilibrium geometries of the metallocenes MCp{sub 2}(M = Ca, Sr, Ba, Sm, Eu, or Yb) have been studied by ab initio pseudopotential calculations at the Hartree-Fock (HF), MP2, and CISD levels. In the HF calculations all molecules are found to favor regular ({open_quotes}linear{close_quotes}) sandwich-type equilibrium structures with increasingly shallow potential energy surfaces for the bending motions along the series M = Ca, Yb, Sr, Eu, Sm, and Ba. Large-scale MP2 calculations for BaCp{sub 2} yield a (ring centroid)-Ba-(ring centroid) angle of ca. 147{degrees}. However, the linearization energy is less than 1.5 kJ/mol. Thus, barocene can be described as a {open_quotes}quasilinear{close_quotes} molecule. The same description seems appropriate for samarocene and europocene. However, CaCp{sub 2} and probably YbCp{sub 2} are genuinely linear, whereas SrCp{sub 2} is intermediate. The low bending potentials of all species studied (even CaCp{sub 2}) permit large-amplitude bending motions. Hence, our computational results for these floppy organometallics can be reconciled with the experimentally observed, bent, thermal average, gas-phase structures of the permethylated metallocenes MCp*{sub 2}(M = Ca, Sr, Ba, or Yb; Cp* = {eta}{sub 5}C{sub 5}Me{sub 5}). As is indicated by natural population analysis (NPA), the largely {pi}-type covalent contributions to M-Cp bonding are responsible for the considerably smaller preference for bent structures compared with. e.g., the corresponding dihalides (MX{sub 2}) or dimethyl compounds (MMe{sub 2}) 37 refs., 2 figs., 6 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 457066
- Journal Information:
- Journal of the American Chemical Society, Vol. 114, Issue 21; Other Information: PBD: 7 Oct 1992
- Country of Publication:
- United States
- Language:
- English
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