X-ray absorption spectroscopy (XAS) has been a powerful tool in deducing the oxidation states of transition metals. Energy shifts in XAS are higher due to the higher effective nuclear charge of atoms in higher oxidations, presumably due to the higher binding energy for deeper, more core-like electrons.
Despite this being a very powerful technique, competing effects on the rising edge positions can make assignment difficult. It was initially thought that the weak, quadrupole-allowed pre-edge peak assigned as the Cu 1s to 3d transition could be used to distinguish between Cu(II) and Cu(III) with the features appearing at 8979 +/- 0.3 eV and 8981 +/- 0.5 eV, respectively. Ab initio calculations by Tomson, Wieghardt, and co-workers displayed that pre-peaks previously assigned as Cu(III) could be displayed by Cu(II) bearing complexes. Many groups have displayed that metal K-edge XAS transitions involving ligand-localised acceptor orbitals, as well as spectral shifts from change in coordination environment, can make metal K-edge analysis less predictable.
The most sussessful use of K and L-edge XAS provide valuable information on the composition of molecular orbitals and display inverted ligand fields has been done in studies that made use of computational techniques in concert with experimental techniques. This was the case of the L[Cu(S)] complexes of York, Brown, and Tolman, and the Cu(CF) by various groups including Hoffman, Overgaard, and Lancaster.
Another experimental tool used to probe ligand field inversion includes Electron paramagnetic resonance (ESR/EPR), which can provide information regarding the metal electronic configuration, the nature of the SOMO, and high resolution information on the ligands.