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Heavy Elements X-ray Absorption Spectroscopies Network

Final Report Summary - HEXANE (Heavy Elements X-ray Absorption Spectroscopies Network)

The hexacyanometallate family is well known in transition metal chemistry because the remarkable electronic delocalization along the metal-cyano-metal bond can be tuned in order to design systems that undergo a controlled change of their physical properties. Heavy elements, and particularly f-transition elements, have a very rich chemistry and their behaviour is still a matter of debate. Actinide elements are often compared to transition metal elements in the sense that their valence 5f orbitals are partially available for chemical bonding, in contrast with lanthanide elements that mainly form pure ionic bonds.
This project aims to fully describe the building block chemistry of hexacyanometallates of heavy elements (as actinides) and to assess the electronic properties of the actinide-cyano bond using a combination of X-ray spectroscopic and theoretical chemical tools.
The main features of the network in HEXANE were
1/ to gather experimentalists and theoreticians from France, Germany, Mexico, Spain and the United States, all interested in structural and electronic interpretation of X-ray absorption spectroscopy of heavy elements,
2/ to involve PhD students as well as experienced permanent staff from each team,
3/ to involve four synchrotron facilities, two in Europe (SOLEIL, ESRF) and two in the United States (ALS, SSRL), giving access to a wide range of photon energies.
During the first period of the project, numerous syntheses of new compounds never described before have been carried out (actinide cyanometallates, lanthanide thiocyanates and Hf cyanoferrate). The compounds have been characterized with classical techniques as FTIR (coordination) and X-ray diffraction when single crystals were obtained. But X-ray Absorption Spectroscopy was the major tool of this project. It focused on structural and electronic characterization with multi edge EXAFS and XANES experiments at the actinide LIII and iron K edge.
In an effort to describe the cyano bridge, a double edge fitting procedure of the EXAFS spectra including both iron and actinide edges and based on multiple scattering approach has been developed. For the first time, the entire series of hexacyanoferrate from Th to Cf has been structurally described at various oxidation states
We have also investigated the electronic properties of these molecular solids. Low energy electronic transitions have been studied: iron LII,III edge, nitrogen and carbon K-edge and also actinide N4,5 edges (NEXAFS range) to directly probe the valence molecular orbitals of the complexes. Because of their sensitivity to iron 3d orbitals, iron LII,III edges are particularly fruitful in order to assess the properties of the cyano bond. Complementary cyano K edge have also been recorded for the first time for this type of materials. Coupling these data with theoretical chemistry and simulation codes like Feff and FDMNES has been implemented. This has led to a semi quantitative description of bond ionocovalency in cyanometallates of actinides.
EXAFS, XANES and NEXAFS data have been recorded at four synchrotron facilities: the ROBL beam line of ESRF for the EXAFS of the actinides, the MARS beam line of SOLEIL for the EXAFS of Lu and Hf and XRS, the 11-2 beam line of SSRL for the EXAFS and XANES of Fe and 11-0-2 of ALS for the NEXAFS measurements.

The electronic description of actinide-cyano bonds was performed in two steps:
- In the hexacyanoferrate complexes all the cyano ligands are structurally equivalent without preferred orientation (Oh symmetry for the iron and D3h for actinides and lanthanides). This geometrical arrangement simplifies the data interpretation for K4Fe(CN)6, ThIV/FeII and NdIII/FeII. The following qualitative interpretation can be drawn for the series:
i- A first qualitative inspection of both C and N K edges shows a very similar evolution: the final state cyano π* MO is very little affected by each atomic character,
ii- The two features of the "cyano" K edge can be attributed to cyano π* splitting,
iii- Negative energy shifts of the main peak of the "cyano" K edge and of the iron L2,3 edges are due to charge transfer effects,
iv- All edges shifts over this homogeneous series of compounds may directly related to the charge of the absorbing atoms. Even if this observation is mostly phenomenological, the electron density transfer from iron to bridging cyano was evident.
- The simulation of the XAS spectra was then performed to corroborate both experimental and theoretical data and obtain a better description of bonds. Simulations were performed on the basis of DFT calculated electronic populations (Mulliken) and DFT optimised structure or EXFAS atomic position for K4[Fe(CN)6] and ThIV/FeII respectively. Simulated spectra were compared to the experimental spectra of K4Fe(CN)6 and ThIV/FeII. The detailed analysis of O2p DOS (density of state) was possible. Although, the iron-cyano π bonding is associated to orbital mixing the corresponding t2g* and π* splitting is too small to be experimentally observed and only one peak is observed (both in simulated and experimental spectra). At higher energy Th/π* MOs are formed due to the covalent interaction of thorium and cyano ligand and are attributed to the second peak.
Using a phenomenological approach, a clear distinctive behaviour between actinides and lanthanides has been shown in this familly of compounds. Then a theoretical approach using quantum chemistry calculation has shown more specifically the effect of covalency in the actinide-ferrocyanide bond. More specifically, π interactions were underlined by both theoretical and experimental methods. This work is a unique attempt to gather structural and electronic information on actinide molecular compounds and on their lanthanide parents.