Trivalent lanthanide ions:: versatile coordination centers with unique spectroscopic and magnetic properties

At first sight, trivalent lanthanide ions Ln(III) are not very attractive to the chemist: the spherical entities with "inner" 4f valence electrons interact electrostatically with their surroundings, display little stereochemical preferences, and have very similar chemical behavior. On the other hand, these ions exhibit rich and unique spectroscopic and magnetic properties that can be taken advantage of either for spectroscopic and magnetic probes, or to construct materials with specific physico-chemical properties. Moreover, the intrinsic chemical drawbacks of the Ln(III) ions can be turned into a benefit since the ions adapt easily to almost any chemical environment and can therefore be readily introduced into a variety of ionic, molecular, and supramolecular edifices where they act as functional centers. We will first outline the historical aspects of Ln(III) coordination chemistry. Fundamental properties of the Ln(III) ions, including coordination numbers and geometries, solvation, hydrolysis and thermodynamic aspects of complexation, are then briefly reviewed. We finally focus on the several methods developed by inorganic chemists to trap the elusive lanthanide ions into environments preserving or even enhancing their physical properties, or increasing the differences in their chemical characteristics. (C) 2000 Elsevier Science S.A. All rights reserved.