The structural and electronic properties of Cu(II) complexes with symmetric tridentate facially coordinating ligands have recently been studied with respect to their Jahn-Teller distortions [1]. In such systems the forces opposing the Jahn-Teller distortion are of two origins, the crystal packing and the rigidity of the ligand systems. The former is difficult to estimate. The latter however can be calculated using the Consistent Force Field [2] method. Calculation of this type have been carried out for several ligands such as tacn (1,4,7-triaminocyclononane), tame (1,1,1-trisaminomethyl-ethane) and tach (1,2,3- cis-triaminocyclohexane) changing the CuN reference bond length along the circular path given by the mexican potential. Figure 1 shows the result obtained for Cu(tacn) 2+ 2 in the more stable optical active configuration. The relative engergies obtained are a measure for the ease of the ligand, to adapt to the new geometry imposed by the Jahn-Teller distortion. defined by the angle θ [3]. The surprising result of these calculations is, that all three ligands stabilize a compressed tetragonal coordination (θ = 60°) by energies in the order of 1 kJ/mol. The lack of any experimental evidence for tetragonal compressed systems, indicate that for Cu 2+ this preference of the ligand systems is overridden by other effects. ▪