Antiferromagnetism in transition-metal complexes. Part IV. Lowlying excited states of binuclear copper(II) carboxylate complexes

Abstract

The singlet–triplet–singlet model describing the anomalous magnetic properties of binuclear copper(II) carboxylates is applied to the available experimental data on some 140 compounds. About one-sixth of the data was eventually rejected because of the poor fit with both this and the Bleaney–Bowers model. The trend in |J|, the singlet–triplet separation, parallels the trend in Δ, the singlet–singlet separation, when the terminal ligand is varied. This indicates that the trends in the σ- and δ-exchange integrals are of the opposite sense. All the exchange integrals are increased nephelauxetically by better bonding of terminal or bridging groups, but, because of the unique geometry of the system, the small component of σ-bonding is decreased overall by radial dispersal of d_z²‘holes’ onto the terminal ligands. This component of σ-bonding is of a direct nature, but no firm conclusions as to the nature of the δ-interaction may be drawn.