- Metal K-edge
Metal K-edge is the excitation of a 1s electron to valence bound states localized on the metal. The K-edge can be divided into the pre-edge region, which comprises the pre-edge and rising edge transitions and the near-edge region, which comprises the intense edge transition and ~150 eV above it.
Pre-edge 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, thus, leading to higher energy of the pre-edge. Bonding interactions with ligands also change the charge on the metal leading to changes in Zeff and, hence, 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, where as 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.
Rising-edge 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 Cu complexes, the rising-edge consists of intense transitions, which provide information about bonding. For Cu(I) 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 Cu(I) complexes can be used to distinguish between two-, three- and four-coordinate Cu(I) sites. In the case of higher oxidation state Cu’s, 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 Cu(III) and some Cu(II) 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. This rising-edge transition can be fit to a valence bond configuration (VBCI) model to obtain the composition of the ground state wavefunction and information on ground state covalency. The VBCI model describes the ground and excited state as a linear combination of the metal-based d-state and the ligand-based charge transfer state. The higher the contribution of the charge transfer state to the ground state, the higher is the ground state covalency indicating stronger metal-ligand bonding.
Near-edge The near-edge region is difficult to quantitatively analyze because it describes transitions to continuum levels that are still under the influence of the core-potential. This region is analogous to the EXAFS region and contains structural information. Extraction of metrical parameters from the edge region can be obtained by using the multiple-scattering code implemented in the MXAN software.
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