Metal K-edge
Metal K-edge spectroscopy is a spectroscopic technique used to study the electronic structures of transition metal atoms and complexes. This method measures X-ray absorption caused by the excitation of a 1s electron to valence bound states localized on the metal, which creates a characteristic absorption peak called the K-edge. The K-edge can be divided into the pre-edge region (comprising the pre-edge and rising edge transitions) and the near-edge region (comprising the intense edge transition and ~150 eV above it).
The K-edge of an open shell transition metal ion displays a weak pre-edge 1s-to-valence-metal-d transition at a lower energy than the intense edge jump. This dipole-forbidden transition gains intensity through a quadrupole mechanism and/or through 4p mixing into the final state. The pre-edge contains information about ligand fields and oxidation state. Higher oxidation of the metal leads to greater stabilization of the 1s orbital with respect to the metal d orbitals, resulting in higher energy of the pre-edge. Bonding interactions with ligands also cause changes in the metal's effective nuclear charge (Zeff), leading to changes in the energy of the pre-edge.
The intensity under the pre-edge transition depends on the geometry around the absorbing metal and can be correlated to the structural symmetry in the molecule. Molecules with centrosymmetry have low pre-edge intensity, whereas the intensity increases as the molecule moves away from centrosymmetry. This change is due to the higher mixing of the 4p with the 3d orbitals as the molecule loses centrosymmetry.
A rising-edge follows the pre-edge, and may consist of several overlapping transitions that are hard to resolve. The energy position of the rising-edge contains information about the oxidation state of the metal.
In the case of copper complexes, the rising-edge consists of intense transitions, which provide information about bonding. For CuI species, this transition is a distinct shoulder and arises from intense electric-dipole-allowed 1s→4p transitions. The normalized intensity and energy of the rising-edge transitions in these CuI complexes can be used to distinguish between two-, three- and four-coordinate CuI sites. In the case of higher-oxidation-state copper atoms, the 1s→4p transition lies higher in energy, mixed in with the near-edge region. However, an intense transition in the rising-edge region is observed for CuIII and some CuII complexes from a formally forbidden two electron 1s→4p+shakedown transition. This “shakedown” process arises from a 1s→4p transition that leads to relaxation of the excited state, followed by a ligand-to-metal charge transfer to the excited state.
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