Spin polarization and ferromagnetic coupling in metallocenium charge transfer complexes

Abstract

Spin polarization effects in metallocenium complexes across the transition metal series Cr, Mn, Fe, Co, Ni are discussed in a qualitative fashion using a restricted open-shell Hartree–Fock approach. The spin polarization is taken into account by a limited configuration interaction (CI) with singly excited configurations. In Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2, the unpaired electrons are localized in molecular orbitals with predominant metal 3d character. The polarization of the metal–cyclopentadiene π bonds then leads to a negative spin density on the cyclopentadiene (Cp) rings. The amount of negative spin density on Cp increases with increasing spin on the metal (S=1/2, S=1, and S=3/2 for ferrocenium, manganocenium, and chromocenium, respectively). The Cp spin density of nickelocenium on the other hand is positive because there is a direct delocalization of spin density to the Cp rings due to the considerable Cp contributions to the e*1g molecular orbitals in which the unpaired electron of nickelocenium is located. Correspondingly, the charge transfer complexes of these compounds with the acceptors tetracyanoethylene (TCNE) or tetracyanoquinodimethane (TCNQ) in an alternating arrangement ...D+A−D+A−... are expected to exhibit ferromagnetic donor–acceptor coupling for Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2 and antiferromagnetic coupling for Ni(C5Me5)+2 according to a model suggested previously by the authors and based on an idea of McConnell. This is in agreement with the experimental results. The observed nuclear magnetic resonance (NMR) shifts of some metallocenes in relation to these findings are discussed.