A DFT Study of the Energetical and Structural Landscape of the Tetrahedral to Square-Planar Conversion of Tetrahalide Complexes of Copper(II)

DFT results and the analysis of the energetic and structural landscape of the title complexes are reported. It is shown that the minima of the ground state potential energy surface occur at positions corresponding to tetragonally compressed structures of D-2d symmetry, with rather distinct electronic stabilization energies in respect to the tetrahedral parent complexes. The structural distortion and the accompanying energetic effects are found to increase with increasing ligand hardness, i.e. from Br- to Cl- to F-. The results are discussed and compared by conventional vibronic coupling and angular overlap model calculations. The Jahn-Teller interaction between the T-2(2) ground state and the 6-vibrational mode of tetrahedral CuX42- splits the T-2(2) ground state; this coupling is supported by 3d-4s mixing, which softens the ground state potential energy surface along the a normal mode [formally a T-2(d(9)) circle plus epsilon circle plus T-2 (d(8)s(1)) pseudo Jahn-Teller coupling interaction in T-d]. The vibronic activity may also occur along the trigonal tau(2) modes, which leads to C-3v- or, if both types of pathway are involved, C-2v- [T-2(2) circle plus (epsilon + tau(2)) coupling] distorted structures, which are very weak as compared to D-2d deformations. Theoretical results are validated and used to explain the structural diversity and electronic spectra reported for CuCl42- in various crystal structures. A strain model, which differentiates between an elastic and a binding component, is qualified for getting an understanding for the appearance of structures from tetrahedral to square planar. In the considered cases, it is generally the elastic strain component, which governs the structural and energetic scene, though the binding strain increment is not negligible sometimes (e.g. Cs2CuCl4). Predictions in respect to corresponding compounds with F- and Br- ligands are made.