13C and 1H NMR spectra of ethyl and t-butyl substituted cobaltocenes dissolved in toluene, benzene and acetone are strongly dependent on concentration and temperature. With increasing metallocene concentration and decreasing temperature almost all 13C and 1H signals of the metallocenes as well as those of the solvents are shifted high field to a different degree. The additional shift is most pronounced for substituted aromatic ring carbons giving rise to a seriously disturbed signal ordering.

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... milar behavior is found for substituted vanadocenes and nickelocenes dissolved in toluene or THF. The effect is attributed to preferred mutual orientation of metallocene and solvent molecules thus inducing additional shifts in the metallocene themselves and in the solvent. From the results it is concluded that neither dipolar nor Fermi contact interaction can satisfactorily account for the effect. Instead it originates from susceptibility shifts which may not be ruled out by internal referencing and which are due to anisotropy and orientation of the molecules. For consequence quantitative comparison of paramagnetic NMR shifts from highly anisotropic molecules is only meaningful if concentrations are known. Measurement of magnetic moments may lead to errors.