Quasilanthanide(III) Behavior of Scandium(III) and Coordination Number

Abstract

Abstract The variable position of scandium(III) with regard to the lanthanides(III) on plots of thermodynamic parameters of complex formation and solubility vs. atomic number of Ln has been explained in terms of coordination number (CN) and effective ionic radii (r M(III)) variations. The emphasis is given to the quasi-heavy-lanthanide behavior of Sc(III) as far as the enthalpies of solution of anhydrous rare earth halides MX3 in water are concerned. It is indicated that this peculiar behavior is due to the constant CN = 6 of Sc(III) in the aqueous phase and in the solid phase of anhydrous ScX3, whereas the CN of heavy lanthanides(III) decreases from 8 in the aquoions to 6 in the crystal lattice of anhydrous LnX3. It appears that in the series of increasing softness, Sc(III) occupies the place between Y(III) and Ln(III): Y(III) < Sc(III) < Ln(III) < An(III). Covalent shortening has a small influence on the “nomadic” behavior of Sc(III). The relative decrease of CN of Ln(III) by 2 with regard to Sc(III), c...