The structure periodicity of [RE(NO3)3(H2O)n]·xH2O salts and aqueous solutions with RE = La–Lu, Y. Systematic characterization based on experiments and calculations

Abstract

On the basis of experimental and calculated data from the literature, a series of compounds and solutions containing rare-earth metal (RE) ions with tetrad and double (`double–double') RE effects were analysed and their features noted. The crystal structures of [RE(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>n</sub>]·xH<sub>2</sub>O complex compounds with RE = La–Lu, Y (the inner sphere is in square brackets) were divided into five types according to the n and x values. For the first time, compounds with n = 5 and x = 1, n = 4 and x = 2, and n = 4 and x = 1 (space group P1) with the largest number of representatives were systematically characterized by average interatomic distances in the first (RE<sup>3+</sup>–O<sup>2−</sup>) and second (RE<sup>3+</sup>–N<sup>5+</sup>) coordination spheres depending on the RE atomic number and the electronegativity difference between RE<sup>3+</sup> and O<sup>2−</sup> (Δχ<sub>RE-O</sub>) and between RE<sup>3+</sup> and N<sup>5+</sup> (Δχ<sub>RE-N</sub>) within the framework of the crystallochemical model. Using the stereoatomic model of crystal structures, it was demonstrated that in [RE(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>n</sub>]·xH<sub>2</sub>O all RE—O bonds are predominantly ionic, the structures of Tm(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O and Tm(NO<sub>3</sub>)<sub>3</sub>·5H<sub>2</sub>O are indistinguishable, and the highly asymmetric RE—O<sub>NO3</sub> bond, starting from Gd, is due to a dispersion-type van der Waals interaction between RE<sup>3+</sup> and N<sup>5+</sup> and the electronic structure of the RE<sup>3+</sup> ion. Analysis of density functional theory calculations (ORCA program) showed an asymmetry in the RE—O<sub>NO3</sub> bonds along a series of rare-earth elements and revealed that, starting from RE = Ho, structures with monodentate coordination of one of the nitrates become more favourable. It was found that `tetrads of compounds' and `solution groups' in the structures of commercial [RE(NO₃)₃(H₂O)_n]·xH₂O salts and their highly dilute aqueous solutions (c = 20 mM) differ, which is caused by the predominantly monodentate coordination of RE with the oxygen atoms of the nitrate groups in solution compared with the bidentate coordination observed in the solid compounds.