Lanthanide

A lanthanide () is any of the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium.

In the periodic table, the first fourteen (up to ytterbium) fill the 4f orbitals. Lutetium (element 71) is also often considered a lanthanide, despite being a d-block element and a transition metal. The IUPAC lists the 15 elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, under the collective name lanthanoid (), which it recommends as chemically more correct for this series. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell. Lutetium is a d-block element (thus also a transition metal), and on this basis its inclusion has been questioned; however, like its congeners scandium and yttrium in group 3, it behaves similarly to the other 14. The term rare-earth element or rare-earth metal is often used to include the stable group 3 elements Sc, Y, and Lu in addition to the 4f elements. All lanthanide elements form trivalent cations, Ln³⁺, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum (La) to lutetium (Lu).

In presentations of the periodic table, the f-block elements are customarily shown as two additional rows below the main body of the table. Despite their abundance, the technical term "lanthanides" is interpreted to reflect a sense of elusiveness on the part of these elements, as it comes from the Greek λανθανειν (lanthanein), "to lie hidden".

The word reflects their property of "hiding" behind each other in minerals. The term derives from lanthanum, first discovered in 1838, at that time a so-called new rare-earth element "lying hidden" or "escaping notice" in a cerium mineral, and it is an irony that lanthanum was later identified as the first in an entire series of chemically similar elements and gave its name to the whole series.

These elements are called lanthanides because the elements in the series are chemically similar to lanthanum. Because "lanthanide" means "like lanthanum", it has been argued that lanthanum cannot logically be a lanthanide, but the International Union of Pure and Applied Chemistry (IUPAC) acknowledges its inclusion based on common usage. However, lanthanide is still commonly used. For many years, mixtures of more than one rare earth were considered to be single elements, such as neodymium and praseodymium being thought to be the single element didymium. Very small differences in solubility are used in solvent and ion-exchange purification methods for these elements, which require repeated application to obtain a purified metal. The diverse applications of refined metals and their compounds can be attributed to the subtle and pronounced variations in their electronic, electrical, optical, and magnetic properties.

The lanthanide metals are soft; their hardness increases across the series. Europium stands out, as it has the lowest density in the series at 5.24 g/cm³ and the largest metallic radius in the series at 208.4 pm. It can be compared to barium, which has a metallic radius of 222 pm. It is believed that the metal contains the larger Eu²⁺ ion and that there are only two electrons in the conduction band. Ytterbium also has a large metallic radius, and a similar explanation is suggested.

{| class="wikitable" style="font-size: 95%; text-align: center;"

|+ Electron configurations and colours of lanthanide ions

|-

!Chemical element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu

|-

| Atomic number

|57||58||59||60||61||62||63||64||65||66||67||68||69||70||71

|-

| Ln³⁺ electron configuration* || — || Orange-yellow || Yellow || Blue-violet || — || — || — || — || Red-brown || Orange-yellow || — || — || — || — || —

|-

| Ln³⁺ ion color in aqueous solution || Colorless || Colorless || Green || Violet || Pink || Pale yellow || Colorless || Colorless || V. pale pink || Pale yellow || Yellow || Rose || Pale green || Colorless || Colorless

|-

| Ln²⁺ ion color in aqueous solution

| style="background:#ddd;"| Oxidation state

| 57 || 58 || 59 || 60 || 61 || 62 || 63 || 64 || 65 || 66 || 67 || 68 || 69 || 70 || 71

|-

| +2|| || || || ||

| style="background:#d00; color: white;"| Sm²⁺ || Eu²⁺

|| || || || ||

| style="background:#d07; color: white;"| Tm²⁺

| style="background:#cf0;"| Yb²⁺ ||

|-

| +3|| La³⁺ || Ce³⁺

| style="background:#cf0;"| Pr³⁺

| style="background:#b0d; color: white;"| Nd³⁺

| style="background:#d0d; color: white;"| Pm³⁺

| style="background:#fe0;"| Sm³⁺

| Eu³⁺ || Gd³⁺

| style="background:#fee;"| Tb³⁺

| style="background:#FFFFC5;"| Dy³⁺

| style="background:#ff0;"| Ho³⁺

| style="background:#e0d; color: white;"| Er³⁺

| style="background:#ef0;"| Tm³⁺

| Yb³⁺ || Lu³⁺

|-

| +4|| || style="background:#fd0;"| Ce⁴⁺

| style="background:#ff0;"| Pr⁴⁺

| style="background:#70d; color: white;"| Nd⁴⁺ || || || ||

| style="background:#d60; color: white;"| Tb⁴⁺

| style="background:#fd0;"| Dy⁴⁺ || || || || ||

|}

Effect of 4f orbitals

Viewing the lanthanides from left to right in the periodic table, the seven 4f atomic orbitals become progressively more filled (see above and ). The electronic configuration of most neutral gas-phase lanthanide atoms is [Xe]6s²4fⁿ, where n is 56 less than the atomic number Z. Exceptions are La, Ce, Gd, and Lu, which have 4fⁿ⁻¹5d¹ (though even then 4fⁿ is a low-lying excited state for La, Ce, and Gd; for Lu, the 4f shell is already full, and the fifteenth electron has no choice but to enter 5d). With the exception of lutetium, the 4f orbitals are chemically active in all lanthanides and produce profound differences between lanthanide chemistry and transition metal chemistry. The 4f orbitals penetrate the [Xe] core and are isolated, and thus they do not participate much in bonding. This explains why crystal field effects are small and why they do not form π bonds. The lanthanide contraction, i.e. the reduction in size of the Ln³⁺ ion from La³⁺ (103 pm) to Lu³⁺ (86.1 pm), is often explained by the poor shielding of the 5s and 5p electrons by the 4f electrons. 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 that has the extra stability of a half-filled shell. Other than Ce(IV) and Eu(II), none of the lanthanides are stable in oxidation states other than +3 in aqueous solution.

In terms of reduction potentials, the Ln^0/3+ couples are nearly the same for all lanthanides, ranging from −1.99 (for Eu) to −2.35 V (for Pr). Thus these metals are highly reducing, with reducing power similar to alkaline earth metals such as Mg (−2.36 V).

Coordination chemistry and catalysis

When in the form of coordination complexes, lanthanides exist overwhelmingly in their +3 oxidation state, although particularly stable 4f configurations can also give +4 (Ce, Pr, Tb) or +2 (Sm, Eu, Yb) ions. All of these forms are strongly electropositive and thus lanthanide ions are hard Lewis acids. The oxidation states are also very stable; with the exceptions of SmI₂ and cerium(IV) salts, lanthanides are not used for redox chemistry. 4f electrons have a high probability of being found close to the nucleus and are thus strongly affected as the nuclear charge increases across the series; this results in a corresponding decrease in ionic radii referred to as the lanthanide contraction.

The low probability of the 4f electrons existing at the outer region of the atom or ion permits little effective overlap between the orbitals of a lanthanide ion and any binding ligand. Thus lanthanide complexes typically have little or no covalent character and are not influenced by orbital geometries. The lack of orbital interaction also means that varying the metal typically has little effect on the complex (other than size), especially when compared to transition metals. Complexes are held together by weaker electrostatic forces which are omni-directional and thus the ligands alone dictate the symmetry and coordination of complexes. Steric factors therefore dominate, with coordinative saturation of the metal being balanced against inter-ligand repulsion. This results in a diverse range of coordination geometries, many of which are irregular, and also manifests itself in the highly fluxional nature of the complexes. As there is no energetic reason to be locked into a single geometry, rapid intramolecular and intermolecular ligand exchange will take place. This typically results in complexes that rapidly fluctuate between all possible configurations.

Many of these features make lanthanide complexes effective catalysts. Hard Lewis acids are able to polarise bonds upon coordination and thus alter the electrophilicity of compounds, with a classic example being the Luche reduction. The large size of the ions coupled with their labile ionic bonding allows even bulky coordinating species to bind and dissociate rapidly, resulting in very high turnover rates; thus excellent yields can often be achieved with loadings of only a few mol%. The lack of orbital interactions combined with the lanthanide contraction means that the lanthanides change in size across the series but that their chemistry remains much the same. This allows for easy tuning of the steric environments and examples exist where this has been used to improve the catalytic activity of the complex and change the nuclearity of metal clusters.

Despite this, the use of lanthanide coordination complexes as homogeneous catalysts is largely restricted to the laboratory and there are currently few examples them being used on an industrial scale. Lanthanides exist in many forms other than coordination complexes and many of these are industrially useful. In particular lanthanide metal oxides are used as heterogeneous catalysts in various industrial processes.

Ln(III) compounds

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, such as the tetra-anion derived from 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA).

:thumb|650px|center|Samples of lanthanide nitrates in their [[hexahydrate form. From left to right: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.]]

Ln(II) and Ln(IV) compounds

The most common divalent derivatives of the lanthanides are for Eu(II), which achieves a favorable f⁷ configuration. Divalent halide derivatives are known for all of the lanthanides. They are either conventional salts or are Ln(III) "electride"-like salts. The simple salts include YbI₂, EuI₂, and SmI₂. The electride-like salts, described as Ln³⁺, 2I⁻, e⁻, include LaI₂, CeI₂ and GdI₂. Many of the iodides form soluble complexes with ethers, e.g. TmI₂(dimethoxyethane)₃. Samarium(II) iodide is a useful reducing agent. Ln(II) complexes can be synthesized by transmetalation reactions. The normal range of oxidation states can be expanded via the use of sterically bulky cyclopentadienyl ligands, in this way many lanthanides can be isolated as Ln(II) compounds.

Ce(IV) in ceric ammonium nitrate is a useful oxidizing agent. The Ce(IV) is the exception owing to the tendency to form an unfilled f shell. Otherwise tetravalent lanthanides are rare. However, recently Tb(IV) and Pr(IV) complexes have been shown to exist.

Hydrides

{| class="wikitable collapsible collapsed" style="font-size:95%;"

|-

!Chemical element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu

|-

| Atomic number

|57||58||59||60||61||62||63||64||65||66||67||68||69||70||71

|-

|Metal lattice (RT)

|dhcp ||fcc ||dhcp ||dhcp ||dhcp ||r ||bcc ||hcp ||hcp ||hcp ||hcp||hcp ||hcp ||hcp ||hcp

|-

|Dihydride

|LaH_2+x||CeH_2+x||PrH_2+x ||NdH_2+x|| ||SmH_2+x ||EuH₂ o <br /> "salt like"||GdH_2+x ||TbH_2+x ||DyH_2+x ||HoH_2+x ||ErH_2+x ||TmH_2+x ||YbH_2+x o, fcc<br /> "salt like" ||LuH_2+x

|-

| style="padding-left: 2em" |Structure

|CaF₂||CaF₂||CaF₂||CaF₂||CaF₂||CaF₂||*PbCl₂ ||CaF₂||CaF₂||CaF₂||CaF₂||CaF₂||CaF₂|| ||CaF₂

|-

| style="padding-left: 2em" |metal sub lattice

|fcc||fcc||fcc||fcc||fcc||fcc||o||fcc||fcc||fcc||fcc||fcc||fcc||o fcc||fcc

|-

|Trihydride ||GdH_3−x||TbH_3−x ||DyH_3−x ||HoH_3−x||ErH_3−x ||TmH_3−x ||||LuH_3−x

|-

|style="padding-left: 2em" |metal sub lattice

|fcc||fcc||fcc||hcp||hcp||hcp||fcc||hcp||hcp||hcp||hcp||hcp||hcp||hcp||hcp

|-

|Trihydride properties<br /> transparent insulators <br />(color where recorded)

|red ||bronze to grey ||PrH_3−x fcc||NdH_3−x hcp ||||golden greenish ||EuH_3−x fcc ||GdH_3−x hcp ||TbH_3−x hcp||DyH_3−x hcp ||HoH_3−x hcp||ErH_3−x hcp ||TmH_3−x hcp ||||LuH_3−x hcp

|}

Lanthanide metals react exothermically with hydrogen to form LnH₂, dihydrides.

!Chemical element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu

|-

| Atomic number

|57||58||59||60||61||62||63||64||65||66||67||68||69||70||71

|-

|Tetrafluoride

| ||CeF₄||PrF₄||NdF₄|| || || || ||TbF₄||DyF₄|| || || || ||

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

| ||white dec||white dec|| || || || || ||white dec || || || || || ||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

|||UF₄ 8||UF₄ 8|| || || || || ||UF₄ 8|| || || || ||

|-

|Trifluoride

|LaF₃||CeF₃||PrF₃||NdF₃||PmF₃||SmF₃||EuF₃||GdF₃||TbF₃||DyF₃||HoF₃||ErF₃||TmF₃||YbF₃||LuF₃

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

|white 1493 ||white 1430||green 1395||violet 1374||pink 1399||white 1306||white 1276||white 1231||white 1172||white 1154||yellow 1143||pink 1140||white 1158||white 1157||white 1182

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

| LaF₃ 9 || LaF₃ 9|| LaF₃ 9|| LaF₃ 9|| LaF₃ 9 ||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8||YF₃ 8

|-

|Trichloride

|LaCl₃||CeCl₃||PrCl₃||NdCl₃||PmCl₃||SmCl₃||EuCl₃||GdCl₃||TbCl₃||DyCl₃||HoCl₃||ErCl₃||TmCl₃||YbCl₃||LuCl₃

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

|white 858||white 817||green 786||mauve 758||purple 786||yellow 682||yellow dec||white 602||white 582||white 647||yellow 720||violet 776||yellow 824||white 865||white 925

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

|UCl₃ 9||UCl₃ 9||UCl₃ 9||UCl₃ 9||UCl₃ 9||UCl₃ 9||UCl₃ 9||UCl₃ 9||PuBr₃ 8 ||PuBr₃ 8||YCl₃ 6||YCl₃ 6||YCl₃ 6||YCl₃ 6||YCl₃ 6

|-

|Tribromide

|LaBr₃||CeBr₃||PrBr₃||NdBr₃||PmBr₃||SmBr₃||EuBr₃||GdBr₃||TbBr₃||DyBr₃||HoBr₃||ErBr₃||TmBr₃||YbBr₃||LuBr₃

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

|white 783||white 733||green 691||violet 682||red 693||yellow 640||grey dec||white 770||white 828||white 879||yellow 919||violet 923||white 954||white dec||white 1025

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

|UCl₃ 9||UCl₃ 9||UCl₃ 9||PuBr₃ 8||PuBr₃ 8||PuBr₃ 8||PuBr₃ 8|| 6|| 6|| 6|| 6|| 6|| 6|| 6|| 6

|-

|Triiodide

|LaI₃||CeI₃||PrI₃||NdI₃||PmI₃||SmI₃||EuI₃||GdI₃||TbI₃||DyI₃||HoI₃||ErI₃||TmI₃||YbI₃||LuI₃

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

|yellow-green 772||yellow 766||green 738||green 784||red 737||orange 850||colorless dec. ||yellow 925 ||brown 957 ||green 978||yellow 994||violet 1015||yellow 1021||white dec||brown 1050

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

|PuBr₃ 8||PuBr₃ 8||PuBr₃ 8||PuBr₃ 8||PuBr₃ 8||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6||BiI₃ 6

|-

|Difluoride

| || || || || ||SmF₂||EuF₂|| || || || || || TmF₂||YbF₂||

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

| || || || || ||purple 1417||yellow 1416|| || || || || || ||grey ||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

| || || || || ||CaF₂ 8||CaF₂ 8|| || || || || || ||CaF₂ 8||

|-

|Dichloride

| || || ||NdCl₂|| ||SmCl₂||EuCl₂|| || ||DyCl₂|| || ||TmCl₂||YbCl₂||

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

| || || ||green 841|| ||brown 859||white 731|| || ||black dec.|| || ||green 718||green 720||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

| || || ||PbCl₂ 9|| ||PbCl₂ 9||PbCl₂ 9|| || ||SrBr₂|| || ||SrI₂ 7||SrI₂ 7 ||

|-

|Dibromide

| || || ||NdBr₂|| ||SmBr₂||EuBr₂|| || ||DyBr₂|| || ||TmBr₂||YbBr₂||

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

| || || ||green 725|| ||brown 669||white 731|| || ||black|| || ||green ||yellow 673||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

| || || ||PbCl₂ 9|| ||SrBr₂ 8||SrBr₂ 8|| || ||SrI₂ 7|| || ||SrI₂ 7||SrI₂ 7 ||

|-

|Diiodide

|LaI₂<br /> metallic||CeI₂<br /> metallic ||PrI₂<br /> metallic ||NdI₂<br /> high pressure metallic || ||SmI₂||EuI₂ ||GdI₂<br /> metallic || ||DyI₂|| || ||TmI₂||YbI₂||

|- style="font-size:85%"

|style="padding-left: 2em" |Color m.p. °C

| ||bronze 808||bronze 758||violet 562|| ||green 520||green 580||bronze 831|| ||purple 721|| || ||black 756||yellow 780||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure C.N.

|CuTi₂ 8||CuTi₂ 8||CuTi₂ 8|| SrBr₂ 8 <br />CuTi₂ 8 || ||EuI₂ 7||EuI₂ 7 ||2H-MoS₂ 6|| || || || ||CdI₂ 6 ||CdI₂ 6 ||

|-

|Ln₇I₁₂

|La₇I₁₂|| ||Pr₇I₁₂|| || || || || ||Tb₇I₁₂|| || || || || ||

|-

|Sesquichloride

|La₂Cl₃ || || || || || || ||Gd₂Cl₃ ||Tb₂Cl₃|| || || Er₂Cl₃ || Tm₂Cl₃ || ||Lu₂Cl₃

|- style="font-size:85%"

|style="padding-left: 2em" |Structure

| || || || || || || ||Gd₂Cl₃ ||Gd₂Cl₃ || || || || || ||

|-

|Sesquibromide

| || || || || || || ||Gd₂Br₃ ||Tb₂Br₃ || || || || || ||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure

| || || || || || || ||Gd₂Cl₃ ||Gd₂Cl₃ || || || || || ||

|-

|Monoiodide

| LaI|| || || || || || || || || || || ||TmI|| ||

|- style="font-size:85%"

|style="padding-left: 2em" |Structure

| NiAs type || || || || || || || || || || || || || ||

|}

The only tetrahalides known are the tetrafluorides of cerium, praseodymium, terbium, neodymium and dysprosium, the last two known only under matrix isolation conditions.

All of the lanthanides form trihalides with fluorine, chlorine, bromine and iodine. They are all high melting and predominantly ionic in nature.

The trihalides were important as pure metal can be prepared from them.

Some of the dihalides are conducting while the rest are insulators. The conducting forms can be considered as Ln^III electride compounds where the electron is delocalised into a conduction band, Ln³⁺ (X⁻)₂(e⁻). All of the diiodides have relatively short metal-metal separations. They dissolve in acids to form salts. A mixed Eu^II/Eu^III oxide Eu₃O₄ can be produced by reducing Eu₂O₃ in a stream of hydrogen. The colors of the sesquisulfides vary metal to metal and depend on the polymorphic form. The colors of the γ-sesquisulfides are La₂S₃, white/yellow; Ce₂S₃, dark red; Pr₂S₃, green; Nd₂S₃, light green; Gd₂S₃, sand; Tb₂S₃, light yellow and Dy₂S₃, orange. The shade of γ-Ce₂S₃ can be varied by doping with Na or Ca with hues ranging from dark red to yellow,

Oxysulfides Ln₂O₂S are well known, they all have the same structure with 7-coordinate Ln atoms, and 3 sulfur and 4 oxygen atoms as near neighbours.

Doping these with other lanthanide elements produces phosphors. As an example, gadolinium oxysulfide, Gd₂O₂S doped with Tb³⁺ produces visible photons when irradiated with high energy X-rays and is used as a scintillator in flat panel detectors.

When mischmetal, an alloy of lanthanide metals, is added to molten steel to remove oxygen and sulfur, stable oxysulfides are produced that form an immiscible solid. Applications in the field of spintronics are being investigated.

The nitrides can be prepared by the reaction of lanthanum metals with nitrogen. Some nitride is produced along with the oxide, when lanthanum metals are ignited in air.

Carbides

Carbides of varying stoichiometries are known for the lanthanides. Non-stoichiometry is common. All of the lanthanides form LnC₂ and Ln₂C₃ which both contain C₂ units.

The dicarbides with exception of EuC₂, are metallic conductors with the calcium carbide structure and can be formulated as Ln³⁺C₂²⁻(e–). The C-C bond length is longer than that in CaC₂, which contains the C₂²⁻ anion, indicating that the antibonding orbitals of the C₂²⁻ anion are involved in the conduction band. These dicarbides hydrolyse to form hydrogen and a mixture of hydrocarbons. EuC₂ and to a lesser extent YbC₂ hydrolyse differently producing a higher percentage of acetylene (ethyne).

The sesquicarbides, Ln₂C₃ can be formulated as Ln₄(C₂)₃. These compounds adopt the Pu₂C₃ structure The C-C bond is less elongated than in the dicarbides, with the exception of Ce₂C₃,

Other carbon rich stoichiometries are known for some lanthanides. Ln₃C₄ (Ho-Lu) containing C, C₂ and C₃ units; Ln₄C₇ (Ho-Lu) contain C atoms and C₃ units and Ln₄C₅ (Gd-Ho) containing C and C₂ units.

Metal rich carbides contain interstitial C atoms and no C₂ or C₃ units. These are Ln₄C₃ (Tb and Lu); Ln₂C (Dy, Ho, Tm) and Ln₃C Applications in the field of spintronics are being investigated. The lanthanide borides are typically grouped together with the group 3 metals with which they share many similarities of reactivity, stoichiometry and structure. Collectively these are then termed the rare earth borides. Producing high purity samples has proved to be difficult.

Dodecaborides, LnB₁₂, are formed by the heavier smaller lanthanides, but not by the lighter larger metals, La – Eu. With the exception YbB₁₂ (where Yb takes an intermediate valence and is a Kondo insulator), the dodecaborides are all metallic compounds. They all have the UB₁₂ structure containing a 3 dimensional framework of cubooctahedral B₁₂ clusters. Analogues of uranocene are derived from dilithiocyclooctatetraene, Li₂C₈H₈. Organic lanthanide(II) compounds are also known, such as Cp*₂Eu. 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.

thumb|A solution of 4% [[Holmium(III) oxide|holmium oxide in 10% perchloric acid, permanently fused into a quartz cuvette as a wavelength calibration standard]]

Crystal field splitting is rather small for the lanthanide ions and is less important than spin–orbit coupling in regard to energy levels. 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. Europium-doped yttrium vanadate was the first red phosphor to enable the development of color television screens. Lanthanide ions have notable luminescent properties due to their unique 4f orbitals. Laporte forbidden f-f transitions can be activated by excitation of a bound "antenna" ligand. This leads to sharp emission bands throughout the visible, NIR, and IR and relatively long luminescence lifetimes.

Occurrence

Samarskite and similar minerals contain lanthanides in association with the elements such as tantalum, niobium, hafnium, zirconium, vanadium, and titanium, from group 4 and group 5, often in similar oxidation states. Monazite is a phosphate of numerous group 3 + lanthanide + actinide metals and mined especially for the thorium content and specific rare earths, especially lanthanum, yttrium and cerium. Cerium and lanthanum as well as other members of the rare-earth series are often produced as a metal called mischmetal containing a variable mixture of these elements with cerium and lanthanum predominating; it has direct uses such as lighter flints and other spark sources which do not require extensive purification of one of these metals. Other lanthanide-bearing minerals include bastnäsite, florencite, chernovite, perovskite, xenotime, cerite, gadolinite, lanthanite, fergusonite, polycrase, blomstrandine, håleniusite, miserite, loparite, lepersonnite, euxenite, all of which have a range of relative element concentration and may be denoted by a predominating one, as in monazite-(Ce). Group 3 elements do not occur as native-element minerals in the fashion of gold, silver, tantalum and many others on Earth, but may occur in lunar soil. Very rare halides of cerium, lanthanum, and presumably other lanthanides, feldspars and garnets are also known to exist.

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 bastnäsite. Monazite sands usually contain all the lanthanide elements, but the heavier elements are lacking in bastnäsite. The lanthanides obey the Oddo–Harkins rule – odd-numbered elements are less abundant than their even-numbered neighbors.

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. Promethium is effectively a man-made element, as all its isotopes are radioactive with half-lives shorter than 20&nbsp;years.

Applications

Industrial

Lanthanide elements and their compounds have many uses but the quantities consumed are relatively small in comparison to other elements. About 15000 ton/year of the lanthanides are consumed as catalysts and in the production of glasses. This 15000 tons corresponds to about 85% of the lanthanide production. From the perspective of value, however, applications in phosphors and magnets are more important. Lanthanide ions are used as the active ions in luminescent materials used in optoelectronics applications, most notably the Nd:YAG laser. Erbium-doped fiber amplifiers are significant devices in optical-fiber communication systems. Phosphors with lanthanide dopants are also widely used in cathode-ray tube technology such as television sets. The earliest color television CRTs had a poor-quality red; europium as a phosphor dopant made good red phosphors possible. Yttrium iron garnet (YIG) spheres can act as tunable microwave resonators.

Lanthanide oxides are mixed with tungsten to improve their high temperature properties for TIG welding, replacing thorium, which was mildly hazardous to work with. Many defense-related products also use lanthanide elements such as night-vision goggles and rangefinders. The SPY-1 radar used in some Aegis equipped warships, and the hybrid propulsion system of s all use rare earth magnets in critical capacities.

The price for lanthanum oxide used in fluid catalytic cracking has risen from $5 per kilogram in early 2010 to $140 per kilogram in June 2011.

Most lanthanides are widely used in lasers, and as (co-)dopants in doped-fiber optical amplifiers; for example, in Er-doped fiber amplifiers, which are used as repeaters in the terrestrial and submarine fiber-optic transmission links that carry internet traffic. 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:

{| class="wikitable" |+ The applications of lanthanides

!Application

!Percentage

|-

|Catalytic converters

|45%

|-

|Petroleum refining catalysts

|25%

|-

|Permanent magnets

|12%

|-

|Glass polishing and ceramics

|7%

|-

|Metallurgical

|7%

|-

|Phosphors

|3%

|-

|Other

|1%

|}

The complex Gd(DOTA) is used in magnetic resonance imaging.

Mixtures containing all of the lanthanides operating as a single-atom catalysts have been proposed for the electroreduction of carbon dioxide (CO₂) to carbon monoxide (CO) with a faradaic efficiency greater than 90%.

Radiation Resistance

Titanium oxides of the lanthanides, , have potential in the storage of nuclear waste. These compounds can incorporate radioactive actinides and yet resist radiation damage. This resistance depends on the critical amorphization temperature of each lanthanide. The critical amorphization temperature decreases with decreasing lanthanide ionic radius. Consequently, lanthanides of a smaller ionic radius possess a higher radiation resistance than larger lanthanides.

Life science

Lanthanide complexes can be used for optical imaging. Applications are limited by the lability of the complexes.

Some applications depend on the unique luminescence properties of lanthanide chelates or cryptates. These are well-suited for this application due to their large Stokes shifts and extremely long emission lifetimes (from microseconds to milliseconds) compared to more traditional fluorophores (e.g., fluorescein, allophycocyanin, phycoerythrin, and rhodamine).

The biological fluids or serum commonly used in these research applications contain many compounds and proteins which are naturally fluorescent. Therefore, the use of conventional, steady-state fluorescence measurement presents serious limitations in assay sensitivity. Long-lived fluorophores, such as lanthanides, combined with time-resolved detection (a delay between excitation and emission detection) minimizes prompt fluorescence interference.

Time-resolved fluorometry (TRF) combined with Förster resonance energy transfer (FRET) offers a powerful tool for drug discovery researchers: Time-Resolved Förster Resonance Energy Transfer or TR-FRET. TR-FRET combines the low background aspect of TRF with the homogeneous assay format of FRET. The resulting assay provides an increase in flexibility, reliability and sensitivity in addition to higher throughput and fewer false positive/false negative results.

This method involves two fluorophores: a donor and an acceptor. Excitation of the donor fluorophore (in this case, the lanthanide ion complex) by an energy source (e.g. flash lamp or laser) produces an energy transfer to the acceptor fluorophore if they are within a given proximity to each other (known as the Förster's radius). The acceptor fluorophore in turn emits light at its characteristic wavelength.

The two most commonly used lanthanides in life science assays are shown below along with their corresponding acceptor dye as well as their excitation and emission wavelengths and resultant Stokes shift (separation of excitation and emission wavelengths).

{| class="wikitable" |align="center" |+ Life Science lanthanide Donor-Acceptor pairings

!Donor

!Excitation⇒Emission λ (nm)

!Acceptor

!Excitation⇒Emission λ (nm)

!Stokes Shift (nm)

|-

|Eu³⁺

|340⇒615

|Allophycocyanin

|615⇒660

|320

|-

|Tb³⁺

|340⇒545

|Phycoerythrin

|545⇒575

|235

|}

Possible medical uses

Currently there is research showing that lanthanide elements can be used as anticancer agents. The main role of the lanthanides in these studies is to inhibit proliferation of the cancer cells. Specifically cerium and lanthanum have been studied for their role as anti-cancer agents.

One of the specific elements from the lanthanide group that has been tested and used is cerium (Ce). There have been studies that use a protein-cerium complex to observe the effect of cerium on the cancer cells. The hope was to inhibit cell proliferation and promote cytotoxicity. Transferrin receptors in cancer cells, such as those in breast cancer cells and epithelial cervical cells, promote the cell proliferation and malignancy of the cancer.

The photobiological characteristics, anticancer, anti-leukemia, and anti-HIV activities of the lanthanides with coumarin and its related compounds are demonstrated by the biological activities of the complex.

Cerium has shown results as an anti-cancer agent due to its similarities in structure and biochemistry to iron. Cerium may bind in the place of iron on to the transferrin and then be brought into the cancer cells by transferrin-receptor mediated endocytosis. The mechanism for this effect is still unclear but it is possible that the lanthanum is acting in a similar way as the cerium and binding to a ligand necessary for cancer cell proliferation.

Biological effects

Due to their sparse distribution in the earth's crust and low aqueous solubility, the lanthanides have a low availability in the biosphere, and for a long time were not known to naturally form part of any biological molecules. In 2007 a novel methanol dehydrogenase that strictly uses lanthanides as enzymatic cofactors was discovered in a bacterium from the phylum Verrucomicrobiota, Methylacidiphilum fumariolicum. This bacterium was found to survive only if there are lanthanides present in the environment. Compared to most other nondietary elements, non-radioactive lanthanides are classified as having low toxicity. The same nutritional requirement has also been observed in Methylorubrum extorquens and Methylobacterium radiotolerans.

See also

• Actinides, the heavier congeners of the lanthanides
• Group 3 element
• Lanthanide probes

Notes

References

Cited sources

External links

• lanthanide Sparkle Model, used in the computational chemistry of lanthanide complexes
• USGS Rare Earths Statistics and Information
• Ana de Bettencourt-Dias: Chemistry of the lanthanides and lanthanide-containing materials
• Eric Scerri, 2007, The periodic table: Its story and its significance, Oxford University Press, New York,