The electronic spectra of two crystal faces of bis(methylphenethylammonium) tetrachlorocuprate(II) are reported. The polarization properties of the transitions suggest the d-orbital energy sequence dx> y2 ≫ dxy> dxz2 > dyz> dz2 for the planar, centrosymmetric CuCl42'ion present in this compound. The d-orbital energies are in agreement with a simple angular-overlap treatment of the bonding in a square-planar complex, except that the dz2 orbital is ~5000 cm-1lower in energy than expected. This depression is rationalized in terms of configuration interaction with the copper 4s orbital. At low temperature, each band in the electronic spectrum exhibits considerable vibrational fine structure. The three lower energy transitions each consist of a single progression, and this is assigned to the algstretching mode. The spacing is in each case ~272 cm-1, and the half-widths of the components are__140 cm-1. The relative intensities of the components in y polarization imply a lengthening of the Cu-Cl bonds of ~8.9 ± 0.3 pm in these excited states, while the overall temperature dependence of the bands suggests that the intensity in each case is probably induced largely by coupling with the out-of-plane bending mode of b2usymmetry with this having an energy of ~ 105 cm-1. The highest energy band in xy polarization consists of four progressions, three of these being much weaker and sharper (half-widths ~30 cm-1) than the fourth (half-width ~120 cm-1). Each of these progressions is also assigned to the algmode with an energy of ~267 cm-1. The relative intensities of the components imply a lengthening of the Cu-Cl bond of ~ 10.3 ± 0.3 pm in this excited state. The separations between the progressions suggest that the bulk of the intensity at 10 K is derived by coupling with the in-plane stretching mode of eusymmetry (energy 188 cm-1), with the three weaker progressions being assigned to coupling with lattice modes of energy 108, 63, and ~20 cm-1. The overall temperature dependence of the band is consistent with this interpretation. The three sharper progressions show a marked broadening of their components as the energy of the peaks increases, and this is rationalized in terms of the different vibrational frequencies expected for the various isotopic isomers expected for the CuCl42'ion. Although the energies of the algmode are lower in the excited electronic states than those in the ground state (276 cm-1), the difference is not as large as might be expected from the changes in the Cu-Cl bond lengths. It is possible that this results from a distortion toward a tetrahedral geometry in the excited electronic states. © 1979, American Chemical Society. All rights reserved.
