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

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), connected with the corresponding decrease of the effective ionic radii of the Ln(III) ions, must also be assumed to explain the quasi-heavy-lanthanide(III) behavior of Sc(III) for the first protonation constant of 1:1 lanthanide(III) nitrilotris(methylenephosphonates). Such a relative decrease of CN of the Ln(III) ions with regard to Sc(III) can even reach 5 and, in consequence, their effective ionic radii can be relatively decreased by about 30%. This would explain the Sc(III) behavior as an ion quasi “lighter”/larger than La(III). In such cases tetrahedral coordination is suggested for the Sc(III) species in some nonaqueous solutions: Sc₂Cl₆ (in 2-butylamine and diethylamine), (CH₃OH)ScI₃, [(CH₃)₂SO]ScCl₃.

Key Words:

• scandium(III)
• yttrium(III)
• lanthanides(III)
• displacements
• coordination number
• effective ionic radii
• scandium(III) as a quasilanthanide(III)
• rare earth trihalides
• solution enthalpies
• solubilities