Hydrated metal ions in aqueous solution: How regular are their structures?

Abstract

The hydration reaction is defined as the transfer of an ion or neutral chemical species from the gaseous phase into water, M(n+)(g) -> M(n+)(aq). In this process, water molecules bind to metal ions through ion-dipole bonds of mainly electrostatic character. The hydration reaction is always strongly exothermic with increasing heat of hydration with increasing charge density of the ion. The structures of the hydrated metal ions in aqueous solution display a variety of configurations depending on the size and electronic properties of the metal ion. The basic configurations of hydrated metal ions in aqueous solution are tetrahedral, octahedral, square antiprismatic, and tricapped trigonal prismatic. This paper gives an overview of the structures of hydrated metal ions in aqueous solution with special emphasis on those with a non-regular coordination figure. Metal ions without d-electrons in the valance shell form regular aqua complexes with a coordination figure, allowing a maximum number of water molecules to be clustered around the metal ion. This number is dependent on the ratio of the metal ion radius to the atomic radius of oxygen in a coordinated water molecule (1.34 angstrom). The lighter lanthanoid(III) ions have a regular tricapped trigonal prismatic configuration with the M-O distance to the capping water molecules somewhat longer than to the prismatic ones. However, with increasing atomic number of the lanthanoid(III) ions, an increasing distortion of the capping water molecules is observed, resulting in a partial loss of water molecules in the capping positions for the heaviest lanthanoids. Metal ions with d(4) and d(9) valance shell electron configuration, as chromium(II) and copper(II), respectively, have Jahn-Teller distorted aqua complexes. Metal ions with low charge and ability to form strong covalent bonds, as silver(I), mercury(II), palladium(II), and platinum(II), often display distorted coordination figures due to the second-order Jahn-Teller effect. Metal ions with d(10)s(2) valence shell electron configuration may have a stereochemically active lone electron pair (hemi-directed complexes) or an inactive one (holo-directed). The hydrated tin(II), lead(II), and thallium(l) ions are hemi-directed in aqueous solution, while the hydrated bismuth(III) ion is bob-directed. The structures of the hydrated cationic oxo-metal ions are reported as well.

Keywords

coordination chemistry; coordination geometry; hydration; metal ions; water

Published in

Pure and Applied Chemistry
2010, volume: 82, number: 10, pages: 1901-1917
Publisher: International Union of Pure and Applied Chemistry

SLU Authors

• Persson, Ingmar

  • Department of Molecular Sciences, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Inorganic Chemistry

Publication identifier

• DOI: https://doi.org/10.1351/PAC-CON-09-10-22

Permanent link to this page (URI)

https://res.slu.se/id/publ/30459