Local structure in americium and californium hexacyanoferrates. Comparison with their lanthanide analogues

Metal hexacyanoferrates are well known molecular solids for a large variety of cations, although very little has been described for actinide adducts. Two new members of actinide(III) hexacyanoferrates have been synthesized with americium and californium cations. They have been structurally characterized by infrared spectroscopy, X-ray diffraction and X-Ray absorption spectroscopy. Combined EXAFS data at the iron K edge and actinide L_III edge lead to propose a three-dimensional model for these two new compounds. Structural data in terms of bond distances have been compared to those reported for the lanthanide(III) hexacyanoferrates. A comparison between the actinide and lanthanide homologues has been carried out, based on ionic radii considerations. Consequently, Am(III)/Nd(III) and Cf(III)/Gd(III) complexes have been compared : the americium adduct with KNdFe^II(CN)₆.4H₂O and the californium adduct with KGdFe^II(CN)₆.3H₂O. Comparison between the present EXAFS data and reported X-ray diffraction data suggests that the americium and neodymium environments are very similar, resulting in a tri-capped trigonal prism polyhedron (CN = 9) where the americium is bonded to six nitrogen atoms and to three water molecules. For the californium adduct, EXAFS derived bond length and angle suggest that the californium cation sits in a bi-capped trigonal prism (CN = 8) as in the gadolinium homologue of known structure. Using EXAFS spectroscopy, this is one of the rare evidences of californium/lanthanide similarities in a molecular compounds. A discussion about actinide(III)/lanthanide(III) comparison and coordination number decrease from Am to Cf is also provided, based on previous crystallographic results reported for the actinide(III) hydrate series