Metal K-edge
Encyclopedia
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 ligand
s 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.
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
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
Open shell
In the context of atomic orbitals, an open shell is a valence shell which is not completely filled with electrons or that has not given all of its valence electrons through chemical bonds with other atoms or molecules during a chemical reaction. Atoms generally reach a noble gas configuration in a...
transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
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
Quadrupole
A quadrupole or quadrapole is one of a sequence of configurations of—for example—electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.-Mathematical...
mechanism and/or through 4p mixing into the final state. The pre-edge contains information about ligand fields and oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
. 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 ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s 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
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
. Molecules with centrosymmetry
Centrosymmetry
The term centrosymmetric, as generally used in crystallography, refers to a space group which contains an inversion center as one of its symmetry elements. In such a space group, for every point in the unit cell there is an indistinguishable point...
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
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than 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.