Metal L-edge

As we move left across the periodic table (e.g. from copper to iron), we create additional holes in the metal 3d orbitals.

For example, a low-spin ferric (FeIII) system in an octahedral environment has a ground state of (t2g)5(eg)0 resulting in transitions to the t2g (dπ) and eg (dσ) sets.

This has the opposite effect on the system, resulting in metal-to-ligand charge transfer, MLCT, and commonly appears as an additional L-edge spectral feature.

To make a quantitative assignment, L-edge data is fitted using a valence bond configuration interaction (VBCI) model where LMCT and MLCT are applied as needed to successfully simulate the observed spectral features.

[3] These simulations are then further compared to density functional theory (DFT) calculations to arrive at a final interpretation of the data and an accurate description of the electronic structure of the complex (Figure 4).

In the case of iron L-edge, the excited state mixing of the metal eg orbitals into the ligand π* make this method a direct and very sensitive probe of backbonding.

Figure 1: L 3 - and L 2 -edges of [CuCl 4 ] 2− .
Figure 2: L-edge spectral components.
Figure 3: Configurations involved in the ground and excited states and the mechanisms by which the intensity of the L-edge features can mix.
Figure 4: Comparison of the Fe L-edges of low-spin K 3 [Fe(CN) 6 ] and [Fe(tacn) 2 ]Cl 3 . Tacn is a σ-only donor, meaning no backbonding and only two main L-edge features. K 3 [Fe(CN) 6 ] has significant backbonding, as is shown by the third transition to higher energy in the L-edge spectrum.