Unusual spectroscopic properties of some chlorocuprate and related metal complexes

The single crystal electronic spectra of several chlorocuprate complexes have been investigated, and these exhibit various anomalous features. In particular, the tetrachlorocuprate spectra provide evidence for the changes in geometry which accompany the \d-d\" transitions and the mechanisms by which they gain intensity. For the planar tetrachlorocuprates band polarisations suggest the d-orbital energy sequence dx 2 -y2 >> dxy > dxzdyz > dz. The temperature dependence of band intensities has been interpreted satisfactorily using vibronic coupling theory and the energies of the intensity inducing vibrations agree well with the infrared data. Unusually low energies are inferred in all complex salts for the infrared and Raman inactive out-of-plane bending vibration of b 2u symmetry. Vibrational fine structure is observed in the low temperature spectra of two planar CuC1 2-4 salts and the change of bond length in the excited states has been estimated by Franck-Condon analysis. One of these spectra provided direct evidence for a breakdown of \"simple\" vibronic theory. The band maxima of all planar tetrachlorocuprates undergo anomalously large red shifts between 10K and 290K and this has been ascribed to a pseudo-tetrahedral distortion of the b 2u potential in the excited states. This distortion has been investigated for the 2Alg (z2 ) excited state using a double minimum potential model and the simulated band shapes and band maxima shifts are in reasonable agreement with experiment. Further evidence for an excited state distorted along the b2u coordinate was obtained from the derivation of a V=1 \"hot\" band system. The effect of a rhombic perturbation to the ligand field was investigated for compounds containing planar CuC1202 and CuC12N2 chromophores. Assignment of the d-orbital energies indicated the sequence dx2-y2 >> dxy > dyz > dxz > dz2 for both. An interesting feature of the spectrum for the CuC12 (H20) 2 complex was the appearance of an anomalous band assigned to coupling with the water 0-H stretching vibrations. This assignment was confirmed from the spectrum of another compound containing the planar CuCl202 chromophore in which the 0-donor is not a water ligand. The spectra of CuC12(H20)2 and a complex of CuC12N2 type exhibited vibrational fine structure and this was explained by the interference of progressions in the a modes. In all the planar complexes the energy of the transition to the dz2 orbital is unusually high and this with the metal-ligand bonding was interpreted using the Angular Overlap Model (AOM) employing both a Cellular Ligand Field approach and a model which considers 2 configuration mixing between the alg(dz2) and a1g(4s) orbitals. The latter method was applied to the interpretation of bonding in a range of CuCl2-4 salts with anions having geometries between planar and 4 highly distorted tetrahedral. The intensities of the allowed transitions over this range of structures were calculated using an AOM based model and satisfactory agreement with the experiment was obtained."