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The lanthanide or lanthanoid (IUPAC nomenclature) series comprises the fourteen elements with atomic numbers 58 through 71, from cerium to lutetium. All lanthanides are f-block elements, corresponding to the filling of the 4f electron shell. Lanthanum, which is a d-block element, may also be considered to be a lanthanide. All lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium.
The lanthanide elements are the group of elements with atomic number increasing from 58 (cerium) to 71 (lutetium). They are termed lanthanide because the lighter elements in the series are chemically similar to lanthanum. Strictly speaking lanthanum is a group 3 element element and the ion La3+ has no f electrons. However this element is often included in any general discussion of the chemistry of the lanthanide elements.
The electronic structure of the lanthanide elements, with minor exceptions is [Xe]6s24fn. In their compounds, the 6s electrons are lost and the ions have the configuration [Xe]4fm. The chemistry of the lanthanides differs from main group elements and transition metals because of the nature of the 4f orbitals. These orbitals are "buried" inside the atom and are shielded from the atom's environment by the 4d and 5p electrons. As a consequence of this the chemistry of the elements is largely determined by their size, which decreases gradually from 102 pm (La3+) with increasing atomic number to 86 pm (Lu3+), the so-called lanthanide contraction. All the lanthanide elements exhibit the oxidation state +3. In addition Ce3+ can lose its single f electron to form Ce4+ with the stable electronic configuration of xenon. Also, Eu3+ can gain an electron to form Eu2+ with the f7 configuration which has the extra stability of a half-filled shell. Promethium is effectively a man-made element as all its isotopes are radioactive with half-lives of less than 20 y.
The similarity in ionic radius between adjacent lanthanide elements makes it difficult to separate them from each other in naturally occurring ores and other mixtures. Historically the very laborious processes of cascading and fractional crystallization was used. Because the lanthanide ions have slightly different radii, the lattice energy of their salts and hydration energies of the ions will be slightly different, leading to a small difference in solubility. Salts of the formula Ln(NO3)3.2NH4NO3.4H2O can be used. Industrially, the elements are separated from each other by solvent extraction. Typically an aqueous solution of nitrates is extracted into kerosene containing tri-n-butylphosphate, (BunO)3PO. The strength of the complexes formed increases as the ionic radius decreases, so solubility in the organic phase increases. Complete separation can be achieved continuously by use of countercurrent exchange methods. The elements can also be separated by ion-exchange chromatography, making use of the fact that the stability constant for formation of EDTA complexes increases for log K ≈ 15.5 for [La(EDTA)]- to log K ≈ 19.8 for [Lu(EDTA)]-. The process, involving two columns, is described in detail in Greenwood & Earnshaw
Ce(IV) is a useful oxidising agent, and Eu(II) is a useful reducing agent. The trivalent lanthanides mostly form ionic salts. The trivalent ions are hard acceptors and form more stable complexes with oxygen-donor ligands than with nitrogen-donor ligands. The larger ions are 9-coordinate in aqueous solution, [Ln(H2O)9]3+ but the smaller ions are 8-coordinate, [Ln(H2O)8]3+. There is some evidence that the later lanthanides have more water molecules in the second coordination sphere. Complexation with monodentate ligands is generally weak because it is difficult to displace water molecules from the first coordination sphere. Stronger complexes are formed with chelating ligands because of the chelate effect.
All the trivalent lanthanide ions, except lutetium, have unpaired f electrons. However the magnetic moments deviate considerably from the spin-only values because of strong spin-orbit coupling. The maximum number of unpaired electrons is 7, in Gd3+, with a magnetic moment of 7.94 B.M., but the largest magnetic moments, at 10.4-10.7 B.M., are exhibited by Dy3+ and Ho3+. However, in Gd3+ all the electrons have parallel spin and this property is important for the use of gadolinium complexes as contrast reagent in MRI scans.
Crystal field splitting is rather small for the lanthanide ions and is less important than spin-orbit coupling in regard to energy levels. Transitions of electrons between f orbitals are forbidden by the Laporte rule. Furthermore, because of the "buried" nature of the f orbitals, coupling with molecular vibrations is weak. Consequently the spectra of lanthanide ions are rather weak and the absorption bands are similarly narrow. Glass containing holmium oxide and holmium oxide solutions (usually in perchloric acid) have sharp optical absorption peaks in the spectral range 200–900 nm and can be used as a wavelength calibration standard for optical spectrophotometers, and are available commercially.
As f-f transitions are Laporte-forbidden, once an electron has been excited, decay to the ground state will be slow. This makes them suitable for use in lasers as it makes the population inversion easy to achieve. The Nd:YAG laser is one that is widely used. Lanthanide ions are also fluorescent as a result of the forbidden nature of f-f transitions. Europium-doped yttrium vanadate was the first red phosphor to enable the development of colour television screens.
Metal-carbon σ bonds are found in alkyls of the lanthanide elements such as [LnMe6]3- and Ln[CH(SiMe3)3]. The cyclopentadiene complexes, of formula [Ln(C5H5)3] and [Ln(C5H5)2Cl] may have η-1, η-2, and η-5 rings. Analogues to uranocene are formed with the cyclo-octadienide ion, C8H82- which is a Hückel's rule aromatic ring.
The trivial name "rare earths" is sometimes used to describe all the lanthanides together with scandium and yttrium. This name arises from the minerals from which they were isolated, which were uncommon oxide-type minerals. However, the use of the name is deprecated by IUPAC, as the elements are neither rare in abundance nor "earths" (an obsolete term for water-insoluble strongly basic oxides of electropositive metals incapable of being smelted into metal using late 18th century technology). Cerium is the 26th most abundant element in the Earth's crust, neodymium is more abundant than gold and even thulium (the least common naturally-occurring lanthanide) is more abundant than iodine. Despite their abundance, even the technical term "lanthanides" could be interpreted to reflect a sense of elusiveness on the part of these elements, as it comes from the Greek λανθανειν (lanthanein), "to lie hidden". However, if not referring to their natural abundance, but rather to their property of "hiding" behind each other in minerals, this interpretation is in fact appropriate. The etymology of the term must be sought in the first discovery of lanthanum, at that time a so-called new rare earth element "lying hidden" in a cerium mineral, but we might call it a fortunate twist of irony that exactly lanthanum was later identified as the first in an entire series of chemically similar elements and could give name to the whole series.
The lanthanide contraction is responsible for the great geochemical divide that splits the lanthanides into light and heavy-lanthanide enriched minerals, the latter being almost inevitably associated with and dominated by yttrium. This divide is reflected in the first two "rare earths" that were discovered: yttria (1794) and ceria (1803). The geochemical divide has put more of the light lanthanides in the Earth's crust, but more of the heavy members in the Earth's mantle. The result is that although large rich ore-bodies are found that are enriched in the light lanthanides, correspondingly large ore-bodies for the heavy members are few. The principal ores are monazite and bastnaesite. Monazite sands usually contain all the lanthanide elements, but the heavier elements are lacking in bastnaesite. The lanthanides obey the Oddo-Harkins rule - odd-numbered elements are less abundant than their even-numbered neighbours.
Three of the lanthanide elements have radioactive isotopes with long half-lives (138La, 147Sm and 176Lu) that can be used to date minerals and rocks from Earth, the Moon and meteorites.
Lanthanides entering the human body due to exposure to various industrial processes can affect metabolic processes. Trivalent lanthanide ions, especially La3+ and Gd3+, can interfere with calcium channels in human and animal cells. Lanthanides can also alter or even inhibit the action of various enzymes.[vague] Lanthanide ions found in neurons can regulate synaptic transmission, as well as block some receptors (for example, glutamate receptors).
Most lanthanides are widely used in lasers. These elements deflect ultraviolet and infrared radiation and are commonly used in the production of sunglass lenses. Other applications are summarized in the following table:
1. ^ the current IUPAC recommendation is that the name lanthanoid be used rather than lanthanide, as the suffix "-ide" is preferred for negative ions whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements. However, lanthanide is still favored in most (~90%) scientific articles and is currently adopted on wikipedia. In the older literature, the name "lanthanon" was often used.
* lanthanide Sparkle Model, used in the computational chemistry of lanthanoid complexes