Variable Photon Energy Photoelectron Spectroscopic Studies of Copper Chlorides: An Experimental Probe of Metal-Ligand Bonding and Changes in Electronic Structure on Ionization

Variable photon energy photoelectron spectra (PES) are reported for the valence band region of cuprous and cupric chlorides for photon energies between 22 and 1253.6 eV. Intensity changes of the PES peaks observed with variation of photon energy are associated with (1) the photoionization cross sections of Cu 3d and Cl 3p atomic orbitals, (2) the Cooper minimum of the Cl 3p orbitals, and (3) resonance effects at the Cu 3p absorption edge. These effects allow a definitive assignment of specific PES features, an experimental estimate of covalent mixing between metal and ligand orbitals for all valence levels, and a quantitative evaluation of relaxation effects on the ionized final states. The intensity changes of the CuCl₄^2- PES peaks are found to be dramatically different from those of CuCl₄^3-, a result of increased covalent mixing in the cupric chlorides. Quantitative analysis of the PES data indicates that in D_4h CuCl₄^2-, the highest energy level, 3b_1g, has 65% Cu 3d character while the antibonding metal levels together have 78% d character. For D_2d CuCl₄^2-, the highest energy level has less covalent mixing than the D_4h salt (68% Cu 3d character), while the mixing averaged over all the metal levels increases (76% Cu 3d). The values for the covalent mixing of the highest energy occupied level are in good agreement with the results from other spectroscopic methods, and variable energy PES provides an additional probe of the covalent mixing averaged over all metal and ligand valence levels. The valence band PES spectra of the cupric chlorides also show satellite peaks with significant intensity out of resonance (∼10% of main band intensities) and large resonance enhancement at the Cu 3p → 3d absorption edge. These data require large final-state relaxation effects which have been interpreted both in terms of SCF-Xα-SW calculations and a Configuration Interaction model. The relaxation results from a large decrease in metal-centered electron-electron repulsion on ionization which stabilizes the d⁸ final state by 6.5 eV. The implications of these results with respect to the redox chemistry of Cu(II) complexes is discussed in terms of covalency effects and metal-centered electron-electron repulsion.