Electronic Structure and Dynamic Jahn-Teller Effect of Cobaltocene from EPR and Optical Studies

In order to determine quantitative information about chemical bonding in metallocenes, we investigated the electronic structure of the cobaltocene molecule by EPR and magnetic susceptibility measurements at 4.2°K and by optical spectroscopy at 77°K. Diamagnetic ruthenocene, weakly paramagnetic nickelocene, and paramagnetic cobaltocene single crystals as well as polycrystalline ferrocene served as host systems. From the poorly resolved optical spectra of pure cobaltocene, approximate ligand field parameters were determined. The magnetic properties (g tensor, cobalt hfs tensor) of the lowest Kramers doublet are explained in terms of the relative magnitudes of (a) spin‐orbit coupling, (b) static orthorhombic distortion, and (c) vibronic coupling (dynamic Jahn‐Teller effect) in the orbitally degenerate ²E_1g ground state. From the analysis of the EPR data of cobaltocene‐doped ruthenocene, we conclude that covalency effects and vibronic interactions (``Ham effect'') are of comparable importance resulting in a drastic modification of the magnetic parameters compared to a free Co²⁺ ion in a static crystal field. In agreement with earlier qualitative and semiquantitative predictions, considerable covalency of the singly occupied $e_{1_g}^{*}$ orbital ($\text{42\%± 5\%}$ ligand, $\text{58\%± 5\%}$ cobalt 3d character) was found. The strong change of the EPR parameters going from the ruthenocene to the ferrocene host lattice originates mainly in a strongly enhanced static orthorhombic splitting parameter in the tighter packed ferrocene environment. In cobaltocene single crystal, magnetic dipole‐dipole interactions broaden the EPR lines beyond detection even at 2°K. Nickelocene, an S =1 case with a large positive zero field splitting, behaves as a pseudodiamagnet at liquid helium temperature; exchange interactions with the cobaltocene dopant cause significant modifications of the g values but leaves the cobalt hfs tensor almost unaffected.