Electronic rearrangements induced by weak intermolecular forces: Orbitally degenerate sandwich complexes

Abstract

The vast majority of all molecules have an orbitally non-degenerate ground state characterized by an electronic wavefunction ψ el depending only weakly on the nuclear coordinates R. In contrast, for orbitally degenerate or near-degenerate molecules quite drastic electronic rearrangements can be induced by relatively weak intermolecular forces provided by nonpolar solvents or molecular solid host lattices. In paramagnetic systems changes in the electronic wavefunction can most conveniently be detected and analyzed by using EPR spectroscopy. Paramagnetic sandwich complexes of the metallocene series M(cp) 2 exhibit two types of orbital degeneracies: Jahn-Teller degeneracies (d 7 systems as Co(cp) 2 and Ni(cp) + 2, low-spin (d 5 systems as Mn(cp) 2 and Fe(cp) + 2) and low-spin/high-spin equilibria (d 5 systems as Mn(cp) 2). By diluting these complexes and ring-substituted derivatives in a large variety of diamagnetic host systems we have been able to control the 6A/ 2E equilibrium of Mn(cp) 2 by influencing the metal-to-ring distance and by changing the degree of ring-alkylation; similarly we have been able to vary the relative weights of the two electronic states contributing to the twofold degenerate electronic ground state of d 5 and d 7 systems to a large degree by variation of the local asymmetric fields offered by the lattice sites of the host systems. In the free gaseous state for all orbitally degenerate d 5 and d 7 metallocenes the Jahn-Teller distortions in the C 5H 5 rings remain entirely dynamic, i.e. ▪ the Jahn-Teller stabilization energy E JT never exceeds the energy of a quantum h v of the corresponding active vibrational modes substantially. The degree of covalent delocalization of the singly occupied degenerate metal 3d orbital over the ligand rings correlates well with Jahn-Teller distortion increasing strongly along the series Fe(cp) + 2 < Mn(cp) 2 < Co(cp) 2 < Ni(cp) + 2. This finding agrees with the predictions of semi-empirical MO calculations. In the solid state, by systematic variation of the host lattice we were able to observe a stepwise transition from a purely dynamic Jahn-Teller effect to the static distortion limit in the case of cobaltocene. Alkyl substituents at the rings influenced the cobaltocene EPR spectra in the same way as asymmetric lattice potentials.