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[[Ligand]] K-edge [[X-ray absorption spectroscopy|XAS]] is a spectroscopic technique that allows the direct study of metal-ligand bonding.<ref>{{Cite doi|10.1016/j.ccr.2004.03.020}}</ref> In a XAS experiment, electrons in ligand 1s orbitals are excited to unfilled p ([[principal quantum number]] n <= 4) and continuum states. This results in a large spectral feature known as a rising edge. Transitions at energies lower than the edge can occur, provided they are to orbitals with some ligand p character, and are called pre-edges. Such transitions are often observed in the data of complexes with metal-ligand bonding. | |||
Pre-edge intensities (D<sub>0</sub>) are related to the amount of ligand (L) character in the unfilled orbital: | |||
<math>D_0(L \ 1s \rightarrow \psi^*) = const \ \vert \langle L \ 1s \vert \mathbf{r} \vert \psi^* \rangle \vert^2 | |||
= \alpha^2 \ const \ \vert \langle L \ 1s \vert \mathbf{r} \vert L \ np \rangle \vert^2 </math> | |||
where <math>\psi^*</math> is the wavefunction of the unfilled orbital, '''r''' is the transition dipole operator, and <math>\alpha^2</math> is the "covalency" or ligand character in the orbital. Since <math>\psi^* = \sqrt{1-\alpha^2} \vert M_d \rangle - \alpha \vert L_{np} \rangle </math>, the above expression relating intensity and quantum transition operators can be simplified to use experimental values: | |||
<math> D_0 = \frac{\alpha^2 h}{3n}I_s</math> | |||
where n is the number of absorbing ligand atoms, h is the number of holes, and I<sub>s</sub> is the transition dipole integral which can be determined experimentally. Therefore, by measuring the intensity of pre-edges, it is possible to experimentally the determine the amount of ligand character in a molecular orbital. | |||
==References== | |||
<references/> | |||
[[Category:Spectroscopy]] | |||
[[Category:Synchrotron-related techniques]] | |||
[[Category:X-rays]] |
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Ligand K-edge XAS is a spectroscopic technique that allows the direct study of metal-ligand bonding.[1] In a XAS experiment, electrons in ligand 1s orbitals are excited to unfilled p (principal quantum number n <= 4) and continuum states. This results in a large spectral feature known as a rising edge. Transitions at energies lower than the edge can occur, provided they are to orbitals with some ligand p character, and are called pre-edges. Such transitions are often observed in the data of complexes with metal-ligand bonding.
Pre-edge intensities (D0) are related to the amount of ligand (L) character in the unfilled orbital:
where is the wavefunction of the unfilled orbital, r is the transition dipole operator, and is the "covalency" or ligand character in the orbital. Since , the above expression relating intensity and quantum transition operators can be simplified to use experimental values:
where n is the number of absorbing ligand atoms, h is the number of holes, and Is is the transition dipole integral which can be determined experimentally. Therefore, by measuring the intensity of pre-edges, it is possible to experimentally the determine the amount of ligand character in a molecular orbital.