Instruct: Given a question, retrieve documents that answer the question.
Query: What is the heaviest lanthanide?
All of the lanthanides form Ln2Q3 (Q= S, Se, Te).[15] The sesquisulfides can be produced by reaction of the elements or (with the exception of Eu2S3) sulfidizing the oxide (Ln2O3) with H2S.[15] The sesquisulfides, Ln2S3 generally lose sulfur when heated and can form a range of compositions between Ln2S3 and Ln3S4. The sesquisulfides are insulators but some of the Ln3S4 are metallic conductors (e.g. Ce3S4) formulated (Ln3+)3 (S2−)4 (e−), while others (e.g. Eu3S4 and Sm3S4) are semiconductors.[15] Structurally the sesquisulfides adopt structures that vary according to the size of the Ln metal. The lighter and larger lanthanides favoring 7-coordinate metal atoms, the heaviest and smallest lanthanides (Yb and Lu) favoring 6 coordination and the rest structures with a mixture of 6 and 7 coordination.[15] Polymorphism is common amongst the sesquisulfides.[52] 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.[53] The shade of γ-Ce2S3 can be varied by doping with Na or Ca with hues ranging from dark red to yellow,[43][53] and Ce2S3 based pigments are used commercially and are seen as low toxicity substitutes for cadmium based pigments.[53]
Lanthanide-carbon σ bonds are well known; however as the 4f electrons have a low probability of existing at the outer region of the atom there is little effective orbital overlap, resulting in bonds with significant ionic character. As such organo-lanthanide compounds exhibit carbanion-like behavior, unlike the behavior in transition metal organometallic compounds. Because of their large size, lanthanides tend to form more stable organometallic derivatives with bulky ligands to give compounds such as Ln[CH(SiMe3)3].[73] Analogues of uranocene are derived from dilithiocyclooctatetraene, Li2C8H8. Organic lanthanide(II) compounds are also known, such as Cp*2Eu.[36]
It has been known since the early 1930s that the salts of certain lanthanides are fluorescent. The reaction of lanthanide salts with nucleic acids was discussed in a number of publications during the 1930s and the 1940s where lanthanum-containing reagents were employed for the fixation of nucleic acid structures. In 1942 complexes of europium, terbium, and samarium were discovered to exhibit unusual luminescence properties when excited by UV light. However, the first staining of biological cells with lanthanides occurred twenty years later when bacterial smears of "E. coli" were treated with aqueous solutions of a europium complex, which under mercury lamp illumination appeared as bright red spots. Attention to lanthanide probes increased greatly in the mid-1970s when Finnish researchers proposed Eu(III), Sm(III), Tb(III), and Dy(III) polyaminocarboxylates as luminescent sensors in time-resolved luminescent (TRL) immunoassays. Optimization of analytical methods from the 1970s onward for lanthanide chelates and time-resolved luminescence microscopy (TRLM) resulted in the use of lanthanide probes in many scientific, medical and commercial fields.
The informal chemical symbol Ln is used in general discussions of lanthanide chemistry to refer to any lanthanide. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell; depending on the source, either lanthanum or lutetium is considered a d-block element, but is included due to its chemical similarities with the other 14.[5] All lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium.
Among the lanthanides, lanthanum is exceptional as it does not have any 4f electrons; indeed, the sudden contraction and lowering of energy of the 4f orbital that is important for the chemistry of the lanthanides only begins to happen at cerium. Thus it is only very weakly paramagnetic, unlike the strongly paramagnetic later lanthanides (with the exceptions of the last two, ytterbium and lutetium, where the 4f shell is completely full).[10] Furthermore, since the melting points of the trivalent lanthanides are related to the extent of hybridisation of the 6s, 5d, and 4f electrons, lanthanum has the second-lowest (after cerium) melting point among all the lanthanides: 920°C.[11] The lanthanides become harder as the series is traversed: as expected, lanthanum is a soft metal. Lanthanum has a relatively high resistivity of 615nΩm at room temperature; in comparison, the value for the good conductor aluminium is only 26.50nΩm.[12][13] Lanthanum is the least volatile of the lanthanides.[14] Like most of the lanthanides, lanthanum has a hexagonal crystal structure at room temperature. At 310°C, lanthanum changes to a face-centered cubic structure, and at 865°C, it changes to a body-centered cubic structure.[13]
A potential application for MOFs is biological imaging and sensing via photoluminescence. A large subset of luminescent MOFs use lanthanides in the metal clusters. Lanthanide photoluminescence has many unique properties that make them ideal for imaging applications, such as characteristically sharp and generally non-overlapping emission bands in the visible and near-infrared (NIR) regions of the spectrum, resistance to photobleaching or 'blinking', and long luminescence lifetimes. However, lanthanide emissions are difficult to sensitize directly because they must undergo LaPorte forbidden f-f transitions. Indirect sensitization of lanthanide emission can be accomplished by employing the "antenna effect," where the organic linkers act as antennae and absorb the excitation energy, transfer the energy to the excited state of the lanthanide, and yield lanthanide luminescence upon relaxation. A prime example of the antenna effect is demonstrated by MOF-76, which combines trivalent lanthanide ions and 1,3,5-benzenetricarboxylate (BTC) linkers to form infinite rod SBUs coordinated into a three dimensional lattice. As demonstrated by multiple research groups, the BTC linker can effectively sensitize the lanthanide emission, resulting in a MOF with variable emission wavelengths depending on the lanthanide identity. Additionally, the Yan group has shown that Eu- and Tb- MOF-76 can be used for selective detection of acetophenone from other volatile monoaromatic hydrocarbons. Upon acetophenone uptake, the MOF shows a very sharp decrease, or quenching, of the luminescence intensity.
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.[79]
Small tonnages of xenotime sand are recovered in association with Malaysian tin mining, etc. and are processed commercially. The lanthanide content is typical of "yttrium earth" minerals and runs about two-thirds yttrium, with the remainder being mostly the heavy lanthanides, where the even-numbered lanthanides (such as Gd, Dy, Er, or Yb) each being present at about the 5% level, and the odd-numbered lanthanides (such as Tb, Ho, Tm, Lu) each being present at about the 1% level. Dysprosium is usually the most abundant of the even-numbered heavies, and holmium is the most abundant of the odd-numbered heavies. The lightest lanthanides are generally better represented in monazite while the heaviest lanthanides are in xenotime.
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 Verrucomicrobia, Methylacidiphilum fumariolicum. This bacterium was found to survive only if there are lanthanides present in the environment.[86] Compared to most other nondietary elements, non-radioactive lanthanides are classified as having low toxicity.[79]
It is relatively difficult to separate ytterbium from other lanthanides due to its similar properties. As a result, the process is somewhat long. First, minerals such as monazite or xenotime are dissolved into various acids, such as sulfuric acid. Ytterbium can then be separated from other lanthanides by ion exchange, as can other lanthanides. The solution is then applied to a resin, which different lanthanides bind in different matters. This is then dissolved using complexing agents, and due to the different types of bonding exhibited by the different lanthanides, it is possible to isolate the compounds.
Actinide Group 3 element Lanthanide contraction Rare earth element Lanthanide probes
Lanthanide triflates consist of a lanthanide metal ion and three triflate ions. The lanthanides, or rare earth metals, are the elements from lanthanum to lutetium in the periodic table. Triflate is a contraction of trifluoromethanesulfonate; its molecular formula is CFSO, and is commonly designated ‘OTf’. Triflic acid is a ‘superacid’ so its conjugate base ions are very stable. Lanthanide triflates are normally nonahydrates, mostly commonly depicted as Ln(OTf)·(HO); however, in the solid state and in aqueous solution, the waters are bound to the lanthanide and the triflates are counteranions, so more accurately lanthanide triflate nonahydrate is written as [Ln(HO)](OTf). Anhydrous lanthanide triflates, Ln(OTf), are also easily obtained as described below. The metal triflate complex is strongly electrophilic, thus acts as a strong Lewis acid.
Similarly to the lanthanide and actinide contractions, there should be a superactinide contraction in the superactinide series where the ionic radii of the superactinides are smaller than expected. In the lanthanides, the contraction is about 4.4 pm per element; in the actinides, it is about 3 pm per element. The contraction is larger in the lanthanides than in the actinides due to the greater localization of the 4f wave function as compared to the 5f wave function. Comparisons with the wave functions of the outer electrons of the lanthanides, actinides, and superactinides lead to a prediction of a contraction of about 2 pm per element in the superactinides; although this is smaller than the contractions in the lanthanides and actinides, its total effect is larger due to the fact that 32 electrons are filled in the deeply buried 5g and 6f shells, instead of just 14 electrons being filled in the 4f and 5f shells in the lanthanides and actinides respectively.
Praseodymium may then be separated from the other lanthanides via ion-exchange chromatography, or by using a solvent such as tributyl phosphate where the solubility of Ln increases as the atomic number increases. If ion-exchange chromatography is used, the mixture of lanthanides is loaded into one column of cation-exchange resin and Cu or Zn or Fe is loaded into the other. An aqueous solution of a complexing agent, known as the eluant (usually triammonium edtate), is passed through the columns, and Ln is displaced from the first column and redeposited in a compact band at the top of the column before being re-displaced by . The Gibbs free energy of formation for Ln(edta·H) complexes increases along the lanthanides by about one quarter from Ce to Lu, so that the Ln cations descend the development column in a band and are fractionated repeatedly, eluting from heaviest to lightest. They are then precipitated as their insoluble oxalates, burned to form the oxides, and then reduced to the metals.
Period 5 has the same number of elements as period 4 and follows the same general structure but with one more post transition metal and one fewer nonmetal. Of the three heaviest elements with biological roles, two (molybdenum and iodine) are in this period; tungsten, in period 6, is heavier, along with several of the early lanthanides. Period 5 also includes technetium, the lightest exclusively radioactive element.
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.
Anhydrous lanthanide triflates can be produced by dehydrating their hydrated counterparts by heating between 180 and 200 °C under reduced pressure for 48 hrs. This is a major advantage of lanthanide triflates compared to lanthanide halides, whose anhydrous forms require more tedious synthetic procedures because they cannot be obtained by dehydrating their hydrates (because of oxyhalide formation).
Period 6 is the first period to include the f-block, with the lanthanides (also known as the rare earth elements), and includes the heaviest stable elements. Many of these heavy metals are toxic and some are radioactive, but platinum and gold are largely inert.
The ionization energies for the lanthanides can be compared with aluminium. In aluminium the sum of the first three ionization energies is 5139 kJ·mol−1, whereas the lanthanides fall in the range 3455 – 4186 kJ·mol−1. This correlates with the highly reactive nature of the lanthanides.
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.[34] It should be noted however, that lanthanides exist in many forms other that coordination complexes and many of these are industrially useful. In particular lanthanide metal oxides are used as heterogeneous catalysts in various industrial processes.
Lanthanide triflates are synthesized from lanthanide oxide and aqueous triflic acid. In a typical preparation, a 1:1 (v/v) solution of trilfic acid in water is added to a slight stoichiometric excess of lanthanide oxide. The mixture is stirred and heated at 100 °C for a few hours, and the excess lanthanide oxide is filtered off. The excess oxide ensures all of the triflic acid is consumed. The water is removed under reduced pressure (or simply boiled away) to leave a hydrated lanthanide triflate, Ln(HO)(OTf).
The results of the increased attraction of the outer shell electrons across the lanthanide period may be divided into effects on the lanthanide series itself including the decrease in ionic radii, and influences on the following or post-lanthanide elements.
EDTA was used in separation of the lanthanide metals by ion-exchange chromatography. Perfected by F. H. Spedding "et al". in 1954, the method relies on the steady increase in stability constant of the lanthanide EDTA complexes with atomic number. Using sulfonated polystyrene beads and Cu as a retaining ion, EDTA causes the lanthanides to migrate down the column of resin while separating into bands of pure lanthanides. The lanthanides elute in order of decreasing atomic number. Due to the expense of this method, relative to countercurrent solvent extraction, ion exchange is now used only to obtain the highest purities of lanthanides (typically greater than 99.99%).
They are called lanthanides because the elements in the series are chemically similar to lanthanum. Both lanthanum and lutetium have been labeled as group 3 elements, because they have a single valence electron in the 5d shell. However, both elements are often included in discussions of the chemistry of lanthanide elements. Lanthanum is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "lanthanide" means "like lanthanum", it has been argued that lanthanum cannot logically be a lanthanide, but IUPAC acknowledges its inclusion based on common usage.[6]
Lanthanide probes are a non-invasive analytical tool commonly used for biological and chemical applications. Lanthanides are metal ions which have their 4f energy level filled and generally refer to elements cerium to lutetium in the periodic table. The fluorescence of lanthanide salts is weak because the energy absorption of the metallic ion is low; hence chelated complexes of lanthanides are most commonly used. The term chelate derives from the Greek word for “claw,” and is applied to name ligands, which attach to a metal ion with two or more donor atoms through dative bonds. The fluorescence is most intense when the metal ion has the oxidation state of 3+. Not all lanthanide metals can be used and the most common are: Sm(III), Eu(III), Tb(III), and Dy(III).