Symmetry and vibronic coupling: The stereochemistry and the ground state potential surface of Cu 2+ in five-coordination

Abstract

The stereochemistry and the electronic properties of five-coordinated Cu 2+ complexes are strongly determined by vibronic interactions of the pseudo Jahn-Teller type. The pseudo rotation between the geometric limits of the square pyramid (C 4v) and trigonal bipyramid (D 3h) is analyzed by two alternative and supplementing approaches, starting from the D 3h and C 4v symmetry, respectively. In the latter treatment the totally symmetric modes play an important role. The symmetry relations between and the equivalence of the two approaches are demonstrated. On this basis the wide range of structural and spectroscopic data can be rationalized, if environmental strain is additionally taken into account. Electrostatic ligand-ligand repulsions in addition to the vibronic coupling seem to stabilize the highest possible polyhedron symmetries and establish a modified epikernel principle for pseudo Jahn-Teller unstable ground states.