ESR Studies of Covalent Copper Complexes with a Rhombic Arrangement

A method of calculation of bond parameters is proposed for magnetic molecular orbitals which describe the ${\text{3d}}^9$ configuration in the case of complexes with a rhombic surrounding ($D_{\text{2h}}$ 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 $d_{{\text{3z}}^2{\mathrm{-r}}^2}$ orbital in the ground state which is essentially $d_{x^2{\mathrm{-y}}^2}$ . 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.