Characterization of chemical bonds in bimetallic cyanides using X-ray absorption spectroscopy at L(2,3) edges

X-ray absorption spectroscopy at the L(2,3) edges of 3d transition metals has been used to study the electronic structure of molecular-based magnets with Curie temperatures ranging from 66 to 315 K. These magnets are bimetallic cyanides of the Prussian blue family, constructed by a three-dimensional assembling of -NC-Cr-III-CN-A(II)-units. The chemical selectivity of X-ray absorption spectroscopy allows information to be extracted on each of the two different metal transition ions that carry the spin moments. Using Ligand Field Multiplet calculations, where hybridization is mainly taken into account through configuration interaction, we have been able to reproduce nicely all the features of the divalent 3d ion L(2,3) edges. From the knowledge of the exact ground state, we have determined its electronic structure and relevant parameters, such as the crystal field strength and the spin-orbit coupling. We have separated covalence and charge transfer effects occurring in the bond between the 3d ions and the cyano ligand. From the L(2,3) edges of A(II) divalent ions, we found that the A(II)-NC bond has a weak covalent character with a 10% charge transfer. From the Lr-2,Lr-3 edges of Cr-III, where Cr ions are bonded to the carbon atom of the cyano ligand, we have shown that the Ligand Field Multiplet model is still fully applicable for strongly hybridized bonds. In this case it is essential to separate between covalence and charge transfer and to take the charge transfer into account through pi back-bonding. The precise knowledge of these parameters is essential for the determination of macroscopic characters like magnetic properties.