Ligand field effect on the chemical shift in XANES spectra of Cu(II) compounds

X-Ray absorption near-edge structure (XANES) spectra at the Cu L and K edges have been measured for a series of Cu(II) compounds to clarify the factor affecting the chemical shift in the XANES spectra. The energy positions of the 2p(1/2) --> 3d and 2p(3/2) --> 3d transition peaks in the Cu L-2,L- 3 XANES spectra and the 1s --> 3d transition peak (pre-edge peak) in the Cu K-edge XANES spectra were strongly influenced by the chemical states of the Cu(II), i.e., the coordination geometries (tetrahedral, octahedral and square planar) and ligand electronegativity. In all the spectra, the peak position shifted to higher energy in the order of the spectrochemical series, and the shifts are explained in terms of the change in the ligand field splitting. It is proposed that chemical shifts in the 2p --> 3d and 1s --> 3d transitions can be attributed primarily to changes in the position of the unfilled Cu 3d level. The result demonstrates that chemical shifts in XANES spectra of Cu(II) compounds can be understood in terms of ligand field theory.