Magnetic exchange interactions in tetranuclear copper(II) complexes. Effect of ligand electronegativity

Magnetic susceptibility measurements are reported for a series of tetranuclear copper (II) complexes [Cu4OX6L4](X = Br or Cl; L = Cl, Br, C5H5N, C5H5NO, tetramethylurea, or dimethyl sulphoxide). These data and literature data on other tetranuclear complexes describe a range of magnetic behaviour, varying from strong intramolecular antiferromagnetic interaction, through weak antiferromagnetic interaction, to apparent ferromagnetism. Intermolecular interactions are found to be negligible. The results are used to evaluate current theoretical models for such systems, and are consistent with the existence of a significant symmetric component in the pairwise exchange Hamiltonian.