Tetragonal distortions and vibronic coupling in hexaflouro nickel(II) and copper (II) complexes

Two parameters, axial (R_a) and equatorial (R_e) metal-ligand distances, describe the tetragonal distortions of the NiF^4?₆ and CuF^4?₆ complexes under study. The experimental values of R_a plotted vs R_e exhibit a smooth curve type dependence valid for MF₆ chromophore of solid state compounds. In the model study it becomes explained by analyzing the shape of the adiabatic potential surface (APS) of the form of E_T(R_a, R_e). Around the energy minima the map of numerical values (obtained by the quantum-chemical CNDOUHF and INDOUHF methods) was transformed to an analytic form from which the quantitative characteristics of APS were obtained in a new, simple way. The set of equatorial-axial parameters as well as the set of vibronic parameters were evaluated and discussed in more detail.