X-ray emission spectroscopy can differentiate between very similar ligands in coordination complexes and help characterize their electronic structure.
There are currently several methods for identifying the ligands that are bound to the metal centres in coordination complexes and for extracting information about the bonding. The methods have various limitations, however, such as an inability to distinguish between neighbouring atoms with similar atomic numbers and difficulties in detecting hydrogen. Now Grigory Smolentsev and colleagues have studied1 a spectroscopic method that allows the identification of bound ligands and provides fundamentally important information about the metal–ligand bonding.
The method uses X-ray emission spectroscopy (XES) alongside theoretical analysis from density functional theory (DFT). XES uses X-rays to excite and promote electrons out of core orbitals. Electrons from higher levels fall to fill the holes and measurable energy is released, which is characteristic of the difference in energy between the two orbitals. To show that simple ligands can be differentiated, Smolentsev and colleagues calculated the spectra of [Mn(H2O)6]2+, [Mn(H2O)5OH]+ and [Mn(H2O)5NH3]2+, and analysed the results with the aid of DFT, which can give a picture of the molecular orbitals that are contributing to the spectra.
This allowed them to observe and identify, using molecular orbital theory, the diagnostic peaks seen in the spectra. They were also able to experimentally differentiate between more complex coordination compounds with very structurally similar bidentate ligands.
Smolentsev, G. et al. X-ray emission spectroscopy to study ligand valence orbitals in Mn coordination complexes. J. Am. Chem. Soc. 10.1021/ja808526m (2009).