Lanthanide compounds

Chemical compounds containing a lanthanide element

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Lanthanide compounds are compounds formed by the 15 elements classed as lanthanides. The lanthanides are generally trivalent, although some, such as cerium and europium, are capable of forming compounds in other oxidation states.

## Hydrides

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

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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[1] | LaH2+x | CeH2+x | PrH2+x | NdH2+x |  | SmH2+x | EuH2 o
"salt like" | GdH2+x | TbH2+x | DyH2+x | HoH2+x | ErH2+x | TmH2+x | YbH2+x o, fcc
"salt like" | LuH2+x
Structure | CaF2 | CaF2 | CaF2 | CaF2 | CaF2 | CaF2 | *PbCl2[2] | CaF2 | CaF2 | CaF2 | CaF2 | CaF2 | CaF2 |  | CaF2
metal sub lattice | fcc | fcc | fcc | fcc | fcc | fcc | o | fcc | fcc | fcc | fcc | fcc | fcc | o fcc | fcc
Trihydride[1] | LaH3−x | CeH3−x | PrH3−x | NdH3−x |  | SmH3−x | EuH3−x[3] | GdH3−x | TbH3−x | DyH3−x | HoH3−x | ErH3−x | TmH3−x |  | LuH3−x
metal sub lattice | fcc | fcc | fcc | hcp | hcp | hcp | fcc | hcp | hcp | hcp | hcp | hcp | hcp | hcp | hcp
Trihydride properties
transparent insulators
(color where recorded)  | red | bronze to grey[4] | PrH3−x fcc | NdH3−x hcp |  | golden greenish[5] | EuH3−x fcc | GdH3−x hcp | TbH3−x hcp | DyH3−x hcp | HoH3−x hcp | ErH3−x hcp | TmH3−x hcp |  | LuH3−x hcp

Lanthanide metals react exothermically with hydrogen to form LnH2, dihydrides.[1] With the exception of Eu and Yb, which resemble the Ba and Ca hydrides (non-conducting, transparent salt-like compounds),they form black pyrophoric, conducting compounds[6] where the metal sub-lattice is face centred cubic and the H atoms occupy tetrahedral sites.[1] Further hydrogenation produces a trihydride which is non-stoichiometric, non-conducting, more salt like. The formation of trihydride is associated with and increase in 8–10% volume and this is linked to greater localization of charge on the hydrogen atoms which become more anionic (H− hydride anion) in character.[1]

## Hydroxides

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All of the lanthanides form hydroxides, Ln(OH)3. With the exception of lutetium(III) hydroxide, which has a cubic structure, they have the hexagonal UCl3 structure.[7] The hydroxides can be precipitated from solutions of LnIII.[8] They can also be formed by the reaction of the sesquioxide, Ln2O3, with water, but although this reaction is thermodynamically favorable it is kinetically slow for the heavier members of the series.[7] Fajans' rules indicate that the smaller Ln3+ ions will be more polarizing and their salts correspondingly less ionic. The hydroxides of the heavier lanthanides become less basic, for example Yb(OH)3 and Lu(OH)3 are still basic hydroxides but will dissolve in hot concentrated NaOH.[9]

## Halides

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Lanthanide halides[9][10][11][6] Chemical element | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu

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Atomic number | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71
Tetrafluoride |  | CeF4 | PrF4 | NdF4 |  |  |  |  | TbF4 | DyF4 |  |  |  |  |
Color m.p. °C  |  | white dec | white dec |  |  |  |  |  | white dec |  |  |  |  |  |
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
Color m.p. °C  | white 1493[12] | white 1430 | green 1395 | violet 1374 | green 1399 | white 1306 | white 1276 | white 1231 | white 1172 | green 1154 | pink 1143 | pink 1140 | white 1158 | white 1157 | white 1182
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
Color m.p. °C  | white 858 | white 817 | green 786 | mauve 758 | green 786 | yellow 682 | yellow dec | white 602 | white 582 | white 647 | yellow 720 | violet 776 | yellow 824 | white 865 | white 925
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
Color m.p. °C  | white 783 | white 733 | green 691 | violet 682 | green 693 | yellow 640 | grey dec | white 770 | white 828 | white 879 | yellow 919 | violet 923 | white 954 | white dec | white 1025
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
Color m.p. °C  |  | yellow 766 | green 738 | green 784 | green 737 | orange 850 | dec. | yellow 925 | 957 | green 978 | yellow 994 | violet 1015 | yellow 1021 | white dec | brown 1050
Structure C.N.  | 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 |
Color m.p. °C  |  |  |  |  |  | purple 1417 | yellow 1416 |  |  |  |  |  |  | grey |
Structure C.N.  |  |  |  |  |  | CaF2 8 | CaF2 8 |  |  |  |  |  |  | CaF2 8 |
Dichloride |  |  |  | NdCl2 |  | SmCl2 | EuCl2 |  |  | DyCl2 |  |  | TmCl2 | YbCl2 |
Color m.p. °C  |  |  |  | green 841 |  | brown 859 | white 731 |  |  | black dec. |  |  | green 718 | green 720 |
Structure C.N.  |  |  |  | PbCl2 9 |  | PbCl2 9 | PbCl2 9 |  |  | SrBr2 |  |  | SrI2 7 | SrI2 7 |
Dibromide |  |  |  | NdBr2 |  | SmBr2 | EuBr2 |  |  | DyBr2 |  |  | TmBr2 | YbBr2 |
Color m.p. °C  |  |  |  | green 725 |  | brown 669 | white 731 |  |  | black |  |  | green | yellow 673 |
Structure C.N.  |  |  |  | PbCl2 9 |  | SrBr2 8 | SrBr2 8 |  |  | SrI2 7 |  |  | SrI2 7 | SrI2 7 |
Diiodide | LaI2
metallic | CeI2
metallic | PrI2
metallic | NdI2
high pressure metallic |  | SmI2 | EuI2 | GdI2
metallic |  | DyI2 |  |  | TmI2 | YbI2 |
Color m.p. °C  |  | bronze 808 | bronze 758 | violet 562 |  | green 520 | green 580 | bronze 831 |  | purple 721 |  |  | black 756 | yellow 780 | Lu
Structure C.N.  | CuTi2 8 | CuTi2 8 | CuTi2 8 | SrBr2 8
CuTi2 8 |  | EuI2 7 | EuI2 7 | 2H-MoS2 6 |  |  |  |  | CdI2 6 | CdI2 6 |
Ln7I12 | La7I12 |  | Pr7I12 |  |  |  |  |  | Tb7I12 |  |  |  |  |  |
Sesquichloride | La2Cl3 |  |  |  |  |  |  | Gd2Cl3 | Tb2Cl3 |  |  | Er2Cl3 | Tm2Cl3 |  | Lu2Cl3
Structure  |  |  |  |  |  |  |  | Gd2Cl3 | Gd2Cl3 |  |  |  |  |  |
Sesquibromide |  |  |  |  |  |  |  | Gd2Br3 | Tb2Br3 |  |  |  |  |  |
Structure  |  |  |  |  |  |  |  | Gd2Cl3 | Gd2Cl3 |  |  |  |  |  |
Monoiodide | LaI[13] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Structure  | NiAs type |  |  |  |  |  |  |  |  |  |  |  |  |  |

### Tetrahalides

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Cerium(IV) fluoride powder

Of the lanthanide tetrahalides, only the fluorides of cerium, praseodymium and terbium are well characterised.[9]

Neodymium(IV) fluoride and dysprosium(IV) fluoride are also known under matrix conditions.[14]

### Trihalides

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All of the lanthanides form trihalides with fluorine, chlorine, bromine and iodine. They are all high melting and predominantly ionic in nature.[9] The fluorides are only slightly soluble in water and are not sensitive to air, and this contrasts with the other halides which are air sensitive, readily soluble in water and react at high temperature to form oxohalides.[15]

The trihalides were important as pure metal can be prepared from them.[9] In the gas phase the trihalides are planar or approximately planar, the lighter lanthanides have a lower % of dimers, the heavier lanthanides a higher proportion. The dimers have a similar structure to Al2Cl6.[16]

### Dihalides

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Some of the dihalides are conducting while the rest are insulators. The conducting forms can be considered as LnIII 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.[10] The CuTi2 structure of the lanthanum, cerium and praseodymium diiodides along with HP-NdI2 contain 44 nets of metal and iodine atoms with short metal-metal bonds (393-386 La-Pr).[10] these compounds should be considered to be two-dimensional metals (two-dimensional in the same way that graphite is). The salt-like dihalides include those of Eu, Dy, Tm, and Yb. The formation of a relatively stable +2 oxidation state for Eu and Yb is usually explained by the stability (exchange energy) of half filled (f7) and fully filled f14. GdI2 possesses the layered MoS2 structure, is ferromagnetic and exhibits colossal magnetoresistance.[10]

### Lower halides

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The sesquihalides Ln2X3 and the Ln7I12 compounds listed in the table contain metal clusters, discrete Ln6I12 clusters in Ln7I12 and condensed clusters forming chains in the sesquihalides. Scandium forms a similar cluster compound with chlorine, Sc7Cl12[9] Unlike many transition metal clusters these lanthanide clusters do not have strong metal-metal interactions and this is due to the low number of valence electrons involved, but instead are stabilised by the surrounding halogen atoms.[10]

LaI is the only known monohalide. Prepared from the reaction of LaI3 and La metal, it has a NiAs type structure and can be formulated La3+ (I−)(e−)2.[13]

## Oxides

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### Monoxides

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Europium and ytterbium form salt-like monoxides, EuO and YbO, which have a rock salt structure.[8] EuO is ferromagnetic at low temperatures,[9] and is a semiconductor with possible applications in spintronics.[17] A mixed EuII/EuIII oxide Eu3O4 can be produced by reducing Eu2O3 in a stream of hydrogen.[7] Neodymium and samarium also form monoxides, but these are shiny conducting solids,[9] although the existence of samarium monoxide is considered dubious.[7]

### Sesquioxides

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La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu

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All of the lanthanides form sesquioxides, Ln2O3. The lighter (larger) lanthanides adopt a hexagonal 7-coordinate structure while the heavier/smaller ones adopt a cubic 6-coordinate "C-M2O3" structure.[11] All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates.[7] They dissolve in acids to form salts.[8]

### Dioxides

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Lanthanide dioxides, LnO2, are only formed by Ce, Pr and Tb.

### Other oxides

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Praseodymium and terbium also form intermediate-valence oxides of varying stoichiometry. The most stable compound of praseodymium at room temperature is Pr6O11 and the most stable of compound of terbium at room temperature is Tb4O7. Cerium can also form intermediate-valence oxides such as Ce6O11 and Ce4O7.

## Chalcogenides

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All of the lanthanides form Ln2Q3 (Q= S, Se, Te).[8] The sesquisulfides can be produced by reaction of the elements or (with the exception of Eu2S3) sulfidizing the oxide (Ln2O3) with H2S.[8] 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.[8] 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.[8]

Polymorphism is common amongst the sesquisulfides.[18] 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.[19] The shade of γ-Ce2S3 can be varied by doping with Na or Ca with hues ranging from dark red to yellow,[10][19] and Ce2S3 based pigments are used commercially and are seen as low toxicity substitutes for cadmium based pigments.[19]

All of the lanthanides form monochalcogenides, LnQ, (Q= S, Se, Te).[8] The majority of the monochalcogenides are conducting, indicating a formulation LnIIIQ2−(e-) where the electron is in conduction bands. The exceptions are SmQ, EuQ and YbQ which are semiconductors or insulators but exhibit a pressure induced transition to a conducting state.[18] Compounds LnQ2 are known but these do not contain LnIV but are LnIII compounds containing polychalcogenide anions.[20]

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.[21] 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.[22] 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.[8]

## Pnictides

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### Nitrides

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All of the lanthanides form a mononitride, LnN, with the rock salt structure. The mononitrides have attracted interest because of their unusual physical properties. SmN and EuN are reported as being "half metals".[10] NdN, GdN, TbN and DyN are ferromagnetic, SmN is antiferromagnetic.[23] Applications in the field of spintronics are being investigated.[17] CeN is unusual as it is a metallic conductor, contrasting with the other nitrides also with the other cerium pnictides. A simple description is Ce4+N3− (e–) but the interatomic distances are a better match for the trivalent state rather than for the tetravalent state. A number of different explanations have been offered.[24] 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.[8] Alternative methods of synthesis are a high temperature reaction of lanthanide metals with ammonia or the decomposition of lanthanide amides, Ln(NH2)3. Achieving pure stoichiometric compounds, and crystals with low defect density has proved difficult.[17] The lanthanide nitrides are sensitive to air and hydrolyse producing ammonia.[6]

### Other pnictides

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The other pnictides phosphorus, arsenic, antimony and bismuth also react with the lanthanide metals to form monopnictides, LnQ, where Q = P, As, Sb or Bi. Additionally a range of other compounds can be produced with varying stoichiometries, such as LnP2, LnP5, LnP7, Ln3As, Ln5As3 and LnAs2.[25]

## Carbides

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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.[26] EuC2 and to a lesser extent YbC2 hydrolyse differently producing a higher percentage of acetylene (ethyne).[27]

The sesquicarbides, Ln2C3 can be formulated as Ln4(C2)3. These compounds adopt the Pu2C3 structure[10] which has been described as having C22− anions in bisphenoid holes formed by eight near Ln neighbours.[28] The lengthening of the C-C bond is less marked in the sesquicarbides than in the dicarbides, with the exception of Ce2C3.[26] Other carbon rich stoichiometries are known for some lanthanides. Ln3C4 (Ho-Lu) containing C, C2 and C3 units;[29] Ln4C7 (Ho-Lu) contain C atoms and C3 units[30] and Ln4C5 (Gd-Ho) containing C and C2 units.[31] Metal rich carbides contain interstitial C atoms and no C2 or C3 units. These are Ln4C3 (Tb and Lu); Ln2C (Dy, Ho, Tm)[32][33] and Ln3C[10] (Sm-Lu).

## Borides

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### Diborides

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Diborides, LnB2, have been reported for Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. All have the same, AlB2, structure containing a graphitic layer of boron atoms. Low temperature ferromagnetic transitions for Tb, Dy, Ho and Er. TmB2 is ferromagnetic at 7.2 K.[10]

### Tetraborides

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Tetraborides, LnB4, have been reported for all of the lanthanides except EuB4, all have the same UB4 structure. The structure has a boron sub-lattice consists of chains of octahedral B6 clusters linked by boron atoms. The unit cell decreases in size successively from LaB4 to LuB4. The tetraborides of the lighter lanthanides melt with decomposition to LnB6.[34] Attempts to make EuB4 have failed.[35] The LnB4 are good conductors[36] and typically antiferromagnetic.[10]

### Hexaborides

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Lanthanum hexaboride

Hexaborides, LnB6, have been reported for all of the lanthanides. They all have the CaB6 structure, containing B6 clusters. They are non-stoichiometric due to cation defects. The hexaborides of the lighter lanthanides (La – Sm) melt without decomposition, EuB6 decomposes to boron and metal and the heavier lanthanides decompose to LnB4 with exception of YbB6 which decomposes forming YbB12. The stability has in part been correlated to differences in volatility between the lanthanide metals.[34] In EuB6 and YbB6 the metals have an oxidation state of +2 whereas in the rest of the lanthanide hexaborides it is +3. This rationalises the differences in conductivity, the extra electrons in the LnIII hexaborides entering conduction bands. EuB6 is a semiconductor and the rest are good conductors.[10][34] LaB6 and CeB6 are thermionic emitters, used, for example, in scanning electron microscopes.[37]

### Dodecaborides

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Lanthanide dodecaborides, LnB12, are formed by the heavier smaller lanthanides from Gd to Lu. 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.[36]

### Higher borides

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The higher boride LnB66 is known for all lanthanide metals. The composition is approximate as the compounds are non-stoichiometric.[36] They all have similar complex structure with over 1600 atoms in the unit cell. The boron cubic sub lattice contains super icosahedra made up of a central B12 icosahedra surrounded by 12 others, B12(B12)12.[36] Other complex higher borides LnB50 (Tb, Dy, Ho, Er, Tm, Lu) and LnB25 are known (Gd, Tb, Dy, Ho, Er) and these contain boron icosahedra in the boron framework.[36]

## Organolanthanide compounds

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Main article: Organolanthanide chemistry

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].[38] Analogues of uranocene are derived from dilithiocyclooctatetraene, Li2C8H8. Organic lanthanide(II) compounds are also known, such as Cp*2Eu.[39]

## See also

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|  | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu

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## References

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