In this work are reported the theoretical expressions for the [g], hyperfine, and superhyperfine (shf) tensors of a d9 square‐planar complex within a molecular orbital (MO) scheme. These expressions include contributions arising from crystal field and charge transfer excitations calculated up to third and second order perturbations, respectively. This makes the present framework more general than those previously used. Through those expressions we have derived from the experimental EPR and optical data the MO coefficients corresponding to the valence b1g(x2−y2), b2g(xy), and eg(xz,yz) levels and also the core polarization contribution K to the hyperfine tensor for the systems CuCl2−4, CuBr2−4, and CdCl2:Cu2+. The 3d charge obtained for CuCl2−4 is equal to 0.61, 0.83, and 0.85 for the antibonding 3b1g, 2b2g, and 2eg levels, respectively. These figures are much closer to the Xα results by Bencini and Gatteschi [J. Am. Chem. Soc. 105, 5535 (1983)] than to those by Desjardins et al. [J. Am. Chem. Soc. 105, 4590 (1983)]. The σ and π covalency for CuBr2−4 are both higher
than for CuCl2−4 in accord to the lower electronegativity for bromine. However, only for the antibonding 3b1g level of CuBr2−4 have we obtained an electronic charge lying mainly on ligands. The covalency of CdCl2:Cu2+ is smaller than that found for CuCl2−4, a fact associated to a higher metal–ligand distance for the former. Evidence of this statement are also given from the analysis of crystal‐field spectra and isotropic shf constant. The values of K derived for CuCl2−4 (128.1×10−4 cm−1), CuBr2−4 (103.6×10−4 cm−1), and CdCl2:Cu2+ (123.9×10−4 cm−1) point out the dependence of K on the equatorial covalency but also on the existence of axial ligands. The [g] tensor of CuBr2−4 is dominated by the charge transfer contribution while the crystal field one is negative. Finally an analysis of the importance of each one of the involved contributions to the spin‐Hamiltonian parameters is reported for the three systems, together with the results obtained through a full diagonalization within crystal field and charge transfer states.