The singlet–triplet splitting (2J=EST) is calculated in bis-[bromo-(N,N-diethylaminoethanolato)copper(II)], belonging to the group of alkoxo-bridged copper(II) dimers, by a recently proposed ab initio method. The singlet–triplet splitting energy is obtained by a perturbation expansion closely related to Anderson’s formalism. The role of all valence electrons is taken into account and it is shown to be very important in order to obtain a satisfying agreement with the experimental result. Two important facts, from a computational point of view, are tested: influence of the atomic basis set size, and influence of modifying of the ligands. Changes in the basis set of the bridging atoms can induce variations in the results from 10% to 20%. As a final result, the theoretical value of the real complex is calculated to 2J(calc)=−1095 cm−1 and agrees with the experimental one 2J(exp)=−817 cm−1.

The influence of structural and chemical parameters on the singlet–triplet splitting (2J=ΔEST ) in modeled exchange coupled oxygen-bridged dimeric copper (II) complexes is studied, using an ab initio method followed by a perturbation development of the configuration interaction. The effect of varying the Cu–O–Cu′ angle and the Cu–O distance in the Cu2O2 core of modeled dimers is calculated. The results are comparable to the experimental ones. Changes in the geometric coordination around the copper (II) atom and around the oxygen atom of the core are studied. The results show that the magnitude of the exchange coupling strongly depends on both of these factors. Electronegativity effects are studied by varying the counterion in the model and by changing from alkoxo-bridged to hydroxo-bridged copper (II) dimers.

The crystal and molecular structure of the title compound has been determined from single-crystal X-ray diffraction data and refined to a final R value of 0.053 using 2 514 independent reflections. The compound belongs to the space group P4,/n of the tetragonal system with two tetrameric molecules in a unit cell of dimensions a = 17.226(4) and c = 14.667(3) A. The tetrameric unit with a CU404 core is of S4 symmetry. Copper-copper distances are 3.481 (1) (two), 3.259(1) A (four) and Cu-0 distances are 1.945(4) (four), 2.734(4) (four), and 1.977(4) A (four). The co-ordination about the copper atom

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The singlet-triplet (S-T) splitting energy of aromatic N-oxide bridged copper(I1) dimers has been calculated using an ab initio SCF+CI treatment. Modelled molecules have been used to point out the chemical and geometrical factors which are suspected to influence the metal-metal interaction in this series of compounds. The direct substituent effects are very weak, but they govern the geometry of the complexes owing to the intramolecular interactions. The bridging angle and copper-copper distance have an important, but not decisive influence on the exchange coupling. The variation of the copper-oxygen bond length has a drastic effect on the 2J constant. Its shortening in the models leads to very strong antiferromagnetic couplings comparable to the experimental ones. The deviation of the halogen atoms out of the Cu,Oz plane leads to a very strong decrease of the antiferromagnetic interaction. Both effects reproduce a shift of 2J comparable to the experimental range (12000 cm-'l). Using real geometries the calculated S-T energy gaps are in agreement with the experimental ones and with the calculations on modelled molecules.