Spin density distribution in cobaltocene

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Abstract
In order to obtain information on the spin density distribution in cobaltocene (Co(cp)2) at low temperature, a single crystal proton ENDOR study of Co(cp)2 doped Mn(cp)(CO)3 has been carried out. The hyperfine tensors of all ten ring protons have been determined. These data indicate that the molecular symmetry of Co(cp)2 is almost D 5h, i.e. the two cp rings are eclipsed. The isotropic hyperfine couplings reflect approximate C 2v (y) symmetry of the spin density distribution and hence support a groundstate with predominant dxz character proposed earlier on the basis of E.P.R. results. Both isotropic and dipolar proton hyperfine couplings have been used to derive the spin densities at the adjacent carbon centres. These results show, that the isotropic proton coupling is mainly due to polarization of the proton 1s orbitals and that direct contributions from σ-spin density on the cp rings are of minor importance. The spin densities in the carbon 2pz orbitals derived from proton ENDOR data are compared to those calculated from E.P.R. (g-tensor) and HMO results. Discrepancies, which have been found, are, among other reasons, due to the fact that the experimental spin density distribution shows only approximately C 2v (y) symmetry. To account for this low symmetry, orbital mixing of the dxz and dyz orbitals has been considered. The deviation from C 2v (y) symmetry predicted by this extended model is qualitatively correct, however, no quantitative agreement with the experimental results has been obtained. Furthermore, the experimental data are compared with MO calculation at EHMO, INDO, X α and HFS level. Satisfactory agreement has been found concerning the groundstate of the complex, the pronounced covalent character of the singly occupied MO and the small contributions of the cp σ-orbitals to the MO containing the unpaired electron.