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A strategy has been developed to prove negative unpaired electron spin density in the Cp tr orbitals of ferrocenium ions which is crucial for the McConnell-Kollmar-Kahn (MKK) mechanism of ferromagnetic coupling within [Cp*2Fe]+[TCNE]" stacks and-after extension to other sandwich cations-for the design of organometallic ferromagnets in general. To this end, the paramagnetic metallocenium ions [(EtMe4Cp)2M]+ (M = Cr, Mn, Fe, Ni), [(¡'PrCp)2Fe]+, and [(iBuCp)2Fe]+ have been prepared as [PF6]" salts and investigated by NMR spectroscopy. At ambient temperature the 13C and 'H signal shifts cover ranges of almost 1300 and 170 ppm, respectively, with shift signs depending on the position of the nucleus under study and on the metal. For the metallocenium ions with M = Cr, Mn, and Ni the signs of the signal shifts show that the spin density in the Cp orbitals is positive for Ni and negative for Cr and Mn. By contrast, the shift sign criterion fails for Fe. The different dihedral angles between the Ca-C/9 bonds and the Cp orbitals in [( EtMe4Cp)2Fe]+ and the two [(RCp)2Fe]+ derivatives probe the sign of the spin density selectively via hyperconjugation; it is found to be negative. The experiments support the MKK mechanism in all cases, regardless whether the intrastack coupling in [Cp*2M]+[TCNE]~b e ferromagnetic or antiferromagnetic. The NMR data determine the spin distribution within a substituted Cp ligand which should be helpful for the optimization of the magnetic interaction. The restrictions for a quantitative application of the NMR method are discussed.

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