Created by Karen McFarlane Holman, Willamette University and posted on VIPEr ( on June 27, Copyright Karen McFarlane Holman This work is licensed under the Creative Commons Attribution-NonCommerical-ShareAlike 3.0 Unported License. To view a copy of this license visit XAS & LFT: X-ray absorption spectroscopy (XAS) as a tool to investigate ligand field theory

For coordination compounds, how does one measure… Magnitude of d-d splitting? Magnitude of charge transfer (CT) transitions? MO energy levels? Extent of M-L orbital overlap? Electronic environment (oxidation state, Z eff )?

For coordination compounds, how does one measure… Magnitude of d-d splitting? Magnitude of charge transfer (CT) transitions? MO energy levels? Extent of M-L orbital overlap? Electronic environment (oxidation state, Z eff )? X-ray absorption spectroscopy (XAS)can be used to determine all of these things simultaneously!

What is XAS? X-ray High energy photons eject core e - s (1s, 2s, etc.) Absorption Spectroscopy Scan over a spectrum, measure absorbance of photons that eject e - s or fluorescence of photons emitted when valence e - s relax into holes 1s h h 3d 4p Elemental Fe:

Just another spectrometer Stanford Synchrotron Radiation Lightsource Beamline 11-2 Ion Chamber 0 Ion Chamber 1 Ion Chamber 2 Sample Detector standard Slits Although you do need a special photon source…

X-rays emerge from the source: A particle accelerator called a synchrotron Dave, an undergraduate student from Willamette University, collects data at the Advanced Light Source, Lawrence Berkeley National Laboratory.

X-rays reach the sample chamber Look through The leaded glass window:

Ligand 1s Metal d manifold h h Sample Backside of Detector Inside the sample chamber 5d 5 metal complex, D 4h symmetry

Synchrotron The light source for intense, coherent X-rays used for XAS Advanced Photon Source The Advanced Light Source

Origin of each part of an X-ray absorption spectrum Pre-edge, K-edge (XANES), and EXAFS regions TransmissionAbsorption/fluorescence Thank you to Chris Kim, Chapman U., for providing most of this slide!

Some definitions… XANES: X-ray absorption near-edge structure is the region of the XAS spectrum leading up to and at the K-edge of an element. K-edge: The energy required to eject a 1s electron (akin to ionization energy, but for core electrons). Pre-edge features: Peaks in the XANES spectrum corresponding to electronic transitions from core electrons to bound states that occur slightly below the K-edge energy.

Features of XAS Example: Ru(bpy) 3 2+

Element Specific: Edges energies well resolved for different elements

Element Specific Ru

Element Specific N

C

Oxidation State: K-edge energies shift when an element is oxidized or reduced or Z eff changes

Bond lengths: Can be determined via EXAFS (another type of XAS experiment not discussed here)

Identify Ligand: Useful for bioinorganic systems or when identity of ligand is ambiguous

Orbital mixing Extent of M-L overlap can be determined from spectral features in the pre-edge region

Coordination number/symmetry: Can be determined from XANES and EXAFS

In situ experiments are possible: Extremely useful for dilute samples such as probing a metal catalytic site in an enzyme; Often non-destructive

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance EXAMPLE 1:

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance K-edge = 2480 eV S 6+

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance K-edge = 2475 eV S 4+

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance 2GSH GSSG -2H +, -2e - +2H +, +2e -

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance K-edge

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance K-edge

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance Pre-edge

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x Energy (eV) Normalized Absorbance Pre-edge Due to M-L bonding

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d EXAMPLE 2:

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum K-edge energy: oxidation state, local environment

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum Pre-edge Energy: MO energy levels

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Cu 3d x2-y2 continuum Pre-edge peak area: Orbital overlap (covalency of M-L bond)

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Zn 3d x2-y2 continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, Chlorine K-edge XANES Spectra in MCl Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1s Cl 3p Zn 3d x2-y2 continuum No Pre-edge Feature

e g a 1g t 1u e g e g * a 1g a 1g * t 1u t 1u * t 1u a 1g e g +t 2g 4d 5s 5p 6 L (LGOs) M LMCT UV-Vis t 2g nb ML 6 The Full Picture: LMCT transitions are not always measureable with UV-Vis (instrument limitations)

The Full Picture: Energy levels of various valence shell MOs are measurable via XANES. Couple with Density Functional Theory (DFT) calculations.

The Full Picture: Energy levels of various valence shell MOs are measurable via XANES. Couple with Density Functional Theory (DFT) calculations. Plus, you can determine Z eff, oxidation state, and extent of M-L overlap. XANES rules!