Direct observation of covalency in transition metal-ligand bonding by XMCD

The understanding of covalency in the chemical bonding between transition metal ions and anions or molecules (ligands) is critical to a vast number of disciplines. We have recently demonstrated that direct experimental evidence of magnetic moment delocalization, i.e. covalency, onto chloride ions in paramagnetic molecules can be obtained through the observation of XMCD at the chlorine K edges. In this proposal, we will (1) expand this methodology to determine the covalency in the paramagnetic [ReBr6]2– as a function of secondary coordination sphere of the molecular entities, and (2) determine the relative covalency of Cl and P in the mixed ligand trans-[ReCl4(PPh3)2]. Experimentally, this information is accessible from the combined analysis of the XANES and XMCD at the Re L2,3, Br K, Cl K, P K edges – all of which are reachable at ID12. In a nutshell, both subprojects aim at establishing XMCD as a means for assessing covalency parameters in transition metal complexes and materials.