Metal L-edge
Encyclopedia
Metal L-edge XAS is an experimental technique that involves the excitation of a metal 2p electron
to unfilled metal d orbitals (e.g. 3d for first-row transition metals). According to the selection rules
, the transition is formally electric-dipole allowed, which not only makes it more intense than an electric-dipole forbidden metal K pre-edge (1s → 3d transition), but also makes it more feature-rich as the lower required energy (~400-1000 eV scandium to copper) results in a higher-resolution experiment. In the simplest case, that of a cupric (CuII)
complex, the 2p → 3d transition produces a 2p53d10 final state. The 2p5 core hole created in the transition has an orbital angular momentum L=1 which then couples to the spin angular momentum S=1/2 to produce J = 3/2 and J=1/2 final states. These states are directly observable in the L-edge spectrum as the two main peaks (Figure 1). The peak at lower energy (~930 eV) has the greatest intensity and is called the L3-edge while the peak at higher energy (~950 eV) has less intensity and is called the L2-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. Therefore, there are two possible final states: t2g6eg0 or t2g5eg1(Figure 2a). Since the ground-state metal configuration has one hole in the eg orbital set and four holes in the t2g orbital set, an intensity ratio of 1:4 might be expected (Figure 2b). However, this model does not take into account covalent bonding
and, indeed, an intensity ratio of 1:4 is not observed in the spectrum. In the case of iron, the d6 excited state will further split in energy due to d-d electron repulsion (Figure 2c). This splitting is given by the right-hand (high-field) side of the d6 Tanabe-Sugano diagram
and can be mapped onto a theoretical simulation of a L-edge spectrum (Figure 2d). Other factors such as p-d electron repulsion and spin-orbit coupling of the 2p and 3d electrons must also be considered to fully simulate the data. For a ferric system, all of these effects result in 252 initial states and 1260 possible final states that together will comprise the final L-edge spectrum (Figure 2e). Despite all of these possible states, it has been established that in a low-spin ferric system, the lowest energy peak is due to a transition to the t2g hole and the more intense and higher energy (~3.5 eV) peak is to that of the unoccupied eg orbitals.
In most systems, bonding between a ligand and a metal atom can be thought of in terms of metal-ligand covalent bond where the occupied ligand orbitals donates some electron density to the metal. This is commonly known as ligand to metal charge transfer or LMCT
. In some cases, low-lying unoccupied ligand orbitals (π*) can receive back-donation (or backbonding) from the occupied metal orbitals. 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. An example of this feature occurs in low-spin ferric [Fe(CN)6]3-
since CN-
is a ligand that can have back-bonding. While back-bonding is important in the initial state, it would only warrant a small feature in the L-edge spectrum. In fact, it is in the final state where the back bonding π* orbitals are allowed to mix with the very intense eg transition, thus borrowing intensity and resulting in the final dramatic three peak spectrum (Figure 3 and Figure 4).
XAS
, as well as other spectroscopies
, look at the excited state to infer information about the ground state. Thus, spectra of this nature contain many features that have to be accounted for and understood. To make a quantitative assignment, L-edge data is fit using a valence bond configuration interaction (VBCI) model where LMCT and MLCT are applied as needed to successfully simulate the observed spectral features. 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.
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
to unfilled metal d orbitals (e.g. 3d for first-row transition metals). According to the selection rules
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
, the transition is formally electric-dipole allowed, which not only makes it more intense than an electric-dipole forbidden metal K pre-edge (1s → 3d transition), but also makes it more feature-rich as the lower required energy (~400-1000 eV scandium to copper) results in a higher-resolution experiment. In the simplest case, that of a cupric (CuII)
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....
complex, the 2p → 3d transition produces a 2p53d10 final state. The 2p5 core hole created in the transition has an orbital angular momentum L=1 which then couples to the spin angular momentum S=1/2 to produce J = 3/2 and J=1/2 final states. These states are directly observable in the L-edge spectrum as the two main peaks (Figure 1). The peak at lower energy (~930 eV) has the greatest intensity and is called the L3-edge while the peak at higher energy (~950 eV) has less intensity and is called the L2-edge.
As we move left across the periodic table (e.g. from copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
to iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
), we create additional holes in the metal 3d orbitals. For example, a low-spin ferric (FeIII) system in an octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
environment has a ground state of (t2g)5(eg)0 resulting in transitions to the t2g (dπ) and eg (dσ) sets. Therefore, there are two possible final states: t2g6eg0 or t2g5eg1(Figure 2a). Since the ground-state metal configuration has one hole in the eg orbital set and four holes in the t2g orbital set, an intensity ratio of 1:4 might be expected (Figure 2b). However, this model does not take into account covalent bonding
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
and, indeed, an intensity ratio of 1:4 is not observed in the spectrum. In the case of iron, the d6 excited state will further split in energy due to d-d electron repulsion (Figure 2c). This splitting is given by the right-hand (high-field) side of the d6 Tanabe-Sugano diagram
Tanabe-Sugano diagram
Tanabe-Sugano diagrams are used in coordination chemistry to predict absorptions in the UV and visible electromagnetic spectrum of coordination compounds. The results from a Tanabe-Sugano diagram analysis of a metal complex can also be compared to experimental spectroscopic data...
and can be mapped onto a theoretical simulation of a L-edge spectrum (Figure 2d). Other factors such as p-d electron repulsion and spin-orbit coupling of the 2p and 3d electrons must also be considered to fully simulate the data. For a ferric system, all of these effects result in 252 initial states and 1260 possible final states that together will comprise the final L-edge spectrum (Figure 2e). Despite all of these possible states, it has been established that in a low-spin ferric system, the lowest energy peak is due to a transition to the t2g hole and the more intense and higher energy (~3.5 eV) peak is to that of the unoccupied eg orbitals.
In most systems, bonding between a ligand and a metal atom can be thought of in terms of metal-ligand covalent bond where the occupied ligand orbitals donates some electron density to the metal. This is commonly known as ligand to metal charge transfer or LMCT
Charge transfer complex
A charge-transfer complex or electron-donor-acceptor complex is an association of two or more molecules, or of different parts of one very large molecule, in which a fraction of electronic charge is transferred between the molecular entities. The resulting electrostatic attraction provides a...
. In some cases, low-lying unoccupied ligand orbitals (π*) can receive back-donation (or backbonding) from the occupied metal orbitals. This has the opposite effect on the system, resulting in metal to ligand charge transfer, MLCT
Charge transfer complex
A charge-transfer complex or electron-donor-acceptor complex is an association of two or more molecules, or of different parts of one very large molecule, in which a fraction of electronic charge is transferred between the molecular entities. The resulting electrostatic attraction provides a...
, and commonly appears as an additional L-edge spectral feature. An example of this feature occurs in low-spin ferric [Fe(CN)6]3-
Ferricyanide
Ferricyanide is the anion [Fe6]3−. It is also called hexacyanoferrate and in rare, but systematic nomenclature, hexacyanidoferrate...
since CN-
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
is a ligand that can have back-bonding. While back-bonding is important in the initial state, it would only warrant a small feature in the L-edge spectrum. In fact, it is in the final state where the back bonding π* orbitals are allowed to mix with the very intense eg transition, thus borrowing intensity and resulting in the final dramatic three peak spectrum (Figure 3 and Figure 4).
XAS
XAS
X-ray absorption spectroscopy is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation sources, which provide intense and tunable X-ray beams. Samples can be in the gas-phase, solution, or...
, as well as other spectroscopies
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, look at the excited state to infer information about the ground state. Thus, spectra of this nature contain many features that have to be accounted for and understood. To make a quantitative assignment, L-edge data is fit using a valence bond configuration interaction (VBCI) model where LMCT and MLCT are applied as needed to successfully simulate the observed spectral features. These simulations are then further compared to density functional theory
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
(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.