A new approach to treating vibronic coupling under stress—The strain-induced enhancement or suppression of Jahn–Teller distortions in tetrahedral CuIICl4-complexes, and the transition to octahedral structures

Abstract Host-lattice and host-site properties play a distinct role in essentially modifying the electronic, but also the elastic properties of molecular compounds and of solids with copper(II), which otherwise underlie the – usually dominating – symmetry and energy effects due to vibronic Jahn–Teller coupling. A symmetry-appropriate theoretical treatment, which closely follows the experiment, is suggested for CuCl 4 2− -complexes. It accounts for the observed widespread appearance of molecular distortions from close-to-tetrahedral to square-planar, and furthermore allows for the extension towards hexa-coordinate species. The significant conclusion is, that the imposed elastic strain plays an important role, whose consideration contributes to a better understanding of hitherto unexplained experimental phenomena, such as the unexpectedly weak dependence of the ground state stabilisation and of the charge-transfer energies on the extent of distortion away from T d .