r understand that lanthanides differ in their properties from the s- and d-block metals; r recall characteristic properties of these elements; r appreciate reasons for their positioning in the Periodic Table; r understand how the size of the lanthanide ions affects certain properties and how this can r understand how to obtain pure samples of individual Ln3+ ions. 1.1 Introduction
Lanthanide chemistry started in Scandinavia. In 1794 Johann Gadolin succeeded in obtaining an ‘earth’(oxide) from a black mineral subsequently known as gadolinite; he called the earth yttria. Soon afterwards, M.H. Klaproth, J.J. Berzelius and W. Hisinger obtained ceria, another earth, from cerite. However, it was not until 1839–1843 that the Swede C.G. Mosander ﬁrst separated these earths into their component oxides; thus ceria was resolved into the oxides of cerium and lanthanum and a mixed oxide ‘didymia’ (a mixture of the oxides of the metals from Pr through Gd). The original yttria was similarly separated into substances called erbia, terbia, and yttria (though some 40 years later, the ﬁrst two names were to be reversed!). This kind of confusion was made worse by the fact that the newly discovered means of spectroscopic analysis permitted misidentiﬁcations, so that around 70 ‘new’ elements were erroneously claimed in the course of the century. Nor was Mendeleev’s revolutionary Periodic Table a help. When he ﬁrst published his Periodic Table in 1869, he was able to include only lanthanum, cerium, didymium (now known to have been a mixture of Pr and Nd), another mixture in the form of erbia, and yttrium; unreliable information about atomic mass made correct positioning of these elements in the table difﬁcult. Some had not yet been isolated as elements. There was no way of predicting how many of these elements there would be until Henry Moseley (1887–1915) analysed the X-ray spectra of elements and gave meaning to the concept of atomic number. He showed that there were 15 elements from lanthanum to lutetium (which had only been identiﬁed in 1907). The discovery of radioactive promethium had to wait until after World War 2. It was the pronounced similarity of the lanthanides to each other, especially each to its neighbours (a consequence of their general adoption of the +3 oxidation state in aqueous solution), that caused their classiﬁcation and eventual separation to be an extremely difﬁcult undertaking. be used in the extraction and separation of the elements;
Lanthanide and Actinide Chemistry S. Cotton C 2006 John Wiley & Sons, Ltd.
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Introduction to the Lanthanides
Subsequently it was not until the work of Bohr and of Moseley that it was known precisely how many of these elements there were. Most current versions of the Periodic Table place lanthanum under scandium and yttrium.
1.2 Characteristics of the Lanthanides
The lanthanides exhibit a number of features in their chemistry that differentiate them from the d-block metals. The reactivity of the elements is greater than that of the transition metals, akin to the Group II metals: 1. A very wide range of coordination numbers (generally 6–12, but numbers of 2, 3 or 4 are known). 2. Coordination geometries are determined by ligand steric factors rather than crystal ﬁeld effects. 3. They form labile ‘ionic’ complexes that undergo facile exchange of ligand. 4. The 4f orbitals in the Ln3+ ion do not participate directly in bonding, being well shielded by the 5s2 and 5p6 orbitals. Their spectroscopic and magnetic properties are thus largely uninﬂuenced by the ligand. 5. Small crystal-ﬁeld splittings and very sharp electronic spectra in comparison with the d-block metals. 6. They prefer anionic ligands with donor atoms of rather high electronegativity (e.g. O, F). 7. They readily form hydrated complexes (on account of the high hydration energy of the small Ln3+ ion) and...