Although vibronic Jahn-Teller interactions almost always cause both 4- and 6-coordinate copper(II) complexes to distort significantly away from regular tetrahedral and octahedral geometries, rather different factors influence the size and nature of the distortions. In the former case, the active mode is a bending vibration and this has a very small force constant which is often influenced by factors such as lattice interactions. The size of the distortion therefore varies widely from one compound to another, sometimes causing the limiting planar geometry to be reached. The very low energy of this bending vibration causes several unusual features to occur in the electronic spectrum of the planar CuCl42-ion. In particular, the significant temperature dependence of the band maxima and the vibrational fine structure observed at low temperature both imply that in the excited electronic states the complex has an equilibrium geometry distorted towards a tetrahedron. For 6-coordinate copper(II), the overall distortion is always rather large, but the fact that the active Jahn-Teller vibration is doubly degenerate, with discrimination between the components only occurring because of higher order effects, means that the geometry can vary from an elongated tetragonal to a compressed tetragonal octahedron relatively easily, the pathway involving orthorhombic intermediates. In this case, the alteration in geometry is accompanied by a change in the electronic ground state wavefunction. When the six ligands are identical, the elongated geometry is almost always more stable, with the unpaired electron in the dx2−y2orbital.