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The recent experimental observation of S=2 EPR signals from tetrameric Cu(II) complexes having the general formula Cu4OX6L4 is interpreted on the basis of a static quasi-Heisenberg model for the complex, where the anisotropic symmetric and skew-symmetric interactions have been taken into account with the approximation of the second order of the perturbation theory. It is found that the observed g value fits the single-ion mean value g=(gCu∥+2gCu⊥)/3, as expected, and the constant for the cubic zero-field splitting may be simply written as B4=(D2a+D2b+D 2c)/120J0, where J0 is the isotropic exchange coupling constant and Da,b,c are the principal values of the symmetric component of the anisotropic interaction. It is found, in contrast, that both the symmetric and skew-symmetric parts may contribute to the not yet observed S=1 signals. When the model is applied to the case of Cu4OCl6(TPPO)4 the B4 value measured by EPR and the J0 value derived by magnetic susceptibility data seem to support a symmetric exchange component very large compared with the dipolar energy.

A method of calculation of bond parameters is proposed for magnetic molecular orbitals which describe the 3d9 configuration in the case of complexes with a rhombic surrounding (D2h symmetry), when the experimental spin Hamiltonian is known. This treatment contains as a particular case that of Kivelson et al. for square planar structures and allows us to evaluate both the anisotropic presence of unpaired spin at the various ligands, and the mixing of the d3z2−r2 orbital in the ground state which is essentially dx2−y2. Applications of this treatment in calculating the bond parameters in the CuCl2·2H2O and CuF2·2H2O compounds allow a comparison between the covalent characters of the Cu–Cl and Cu–F bonds in a similar surrounding. The results show that there is a relevant spin density at the halide ions in both compounds and that, in CuCl2·2H2O, the anisotropy of covalence in the principal plane of coordination is strong. The reliability of the proposed criteria is considered by a comparison between the covalence values obtained following our method and those obtained directly from the extrahyperfine structure in the case of a square planar copper complex.