ESR study of the electronic structure and dynamic Jahn-Teller effect in nickelocenium cation

The nickelocenium cation doped with the magnetic isotope 61Ni has been diluted into several diamagnetic host lattices and studied by EPR spectroscopy at low temperatures in polycrystalline samples. From the analysis of the 61Ni hyperfine tensor, supplemented by EHMO and MS-Xα calculations, a quantitative comparison of the covalency and the dynamic Jahn-Teller effects with the related d7 system cobaltocene has been possible. From line-width studies as a function of temperature we conclude that the line shape at low temperatures (4 K) is mainly due to inhomogeneous broadening effects; the spin-lattice relaxation increasing rapidly at higher temperatures cannot be explained solely by an Orbach mechanism via a closeby upper Kramers doublet in a satisfactory way.