Høgni Weihe, Hans U. Güdel and Hans Toftlund
Superexchange in Magnetic Insulators: An Interpretation of
the Metal-Metal Charge Transfer Energy in the Anderson Theory
Inorg. Chem. 39, 1351-1362 (2000)
Abstract:
The superexchange interactions in four three-center model systems A-L-B, for
A and B being paramagnetic centers and L a diamagnetic bridging ligand, are
analyzed by valence bond configuration interaction models in combination with
fourth-order perturbation theory. We analyze the four distinct cases where a
bridging ligand orbital simultaneously interacts with half-filled orbitals
localized on A and B (case i), a half-filled orbital localized on A and an empty
orbital localized on B (case ii), a full orbital localized on A and a
half-filled orbital localized on B (case iii), and finally a full orbital
localized on A and an empty orbital localized on B (case iv). In all four cases
we compare our new results using localized orbitals with the equivalent results
obtained using the Anderson ansatz of delocalized (magnetic) orbitals. The
effective metal-to-metal electron transfer energy Ueff in the
old formalism with delocalized orbitals is expressed in terms of the
metal-to-metal electron transfer energy U and the ligand-to-metal
electron transfer energy D using
localized orbitals. We find that the old formalism containing only
Ueff is in general not sufficient. For cases i and ii we show
that Ueff can be regarded as an effective U strongly
reduced with respect to the free ion as a result of hybridization effects,
whereas the same reduction of U for the cases iii and iv is not possible.
The relevance and applicability of our theoretical results is demonstrated on
magnetochemical data from the literature.