The Scandium Aqua Ion Revisited

Abstract

X-ray diffraction studies on scandium salts in solution and in the solid state, have afforded Sc-O bond lengths in the ions [Sc(H2O)n]³⁺ (n = 6, 7 and 8) as well as for several compounds containing [(H₂O)₅Sc(μ-OH)₂Sc(H₂O)₅]⁴⁺ ions. Several theoretical studies have examined different models for the aqua ion without finding agreement. This review considers these studies. Diffraction results from solutions yield bond lengths which are compared with species of known geometry and coordination number in the solid state, and with different quantum mechanical predictions. On balance, it is considered that a seven-coordinate [Sc(OH2)7]³⁺ ion is the species found in solution.

GRAPHICAL ABSTRACT

Keywords:

• scandium(iii)
• lanthanide(iii)
• aqua ion
• coordination number
• ionic radius

ACKNOWLEDGMENTS

I am very grateful to Professor Roy Johnston for helpful discussions, and to the Comments on Inorganic Chemistry editors—Professors Rossana Galassi, Thomas R. Cundari, and Mohammad A. Omary (and his student, Kortney M. Melançon)—for their very considerable assistance in improving the graphics and feedback on the manuscript content during the preparation of this article.