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User:Praseodymium-141/Lanthanide compounds 2

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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, Tb) or +2 (Eu, Yb) ions. All of these forms are strongly electropositive and thus lanthanide ions are hard Lewis acids.[1] The oxidation states are also very stable; with the exceptions of SmI2[2] and cerium(IV) salts,[3] 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,[4] 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%.[5] 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[6][7][8] and change the nuclearity of metal clusters.[9][10]

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.[11] 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

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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.[12] 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).

 
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.

Compounds in other oxidation states

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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.[13] 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.[14] 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)[15][16][17] and Pr(IV)[18] complexes have been shown to exist.

Hydrides

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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[19] 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[20] 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[19] LaH3−x CeH3−x PrH3−x NdH3−x SmH3−x EuH3−x[21] 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[22] PrH3−x fcc NdH3−x hcp golden greenish[23] 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.[19] 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[24] where the metal sub-lattice is face centred cubic and the H atoms occupy tetrahedral sites.[19] 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.[19]

Halides

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Lanthanide halides[25][26][27][24]
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
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[28] 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[29]
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.[25][30] All of the lanthanides form trihalides with fluorine, chlorine, bromine and iodine. They are all high melting and predominantly ionic in nature.[25] 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.[31]

The trihalides were important as pure metal can be prepared from them.[25] 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.[32]

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.[26] 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).[26] 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[26]

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[25] 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.[26]

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.[29]

Oxides and hydroxides

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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.[27] All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates.[33] They dissolve in acids to form salts.[34]

Cerium forms a stoichiometric dioxide, CeO2, where cerium has an oxidation state of +4. CeO2 is basic and dissolves with difficulty in acid to form Ce4+ solutions, from which CeIV salts can be isolated, for example the hydrated nitrate Ce(NO3)4.5H2O. CeO2 is used as an oxidation catalyst in catalytic converters.[34] Praseodymium and terbium form non-stoichiometric oxides containing LnIV,[34] although more extreme reaction conditions can produce stoichiometric (or near stoichiometric) PrO2 and TbO2.[25]

Europium and ytterbium form salt-like monoxides, EuO and YbO, which have a rock salt structure.[34] EuO is ferromagnetic at low temperatures,[25] and is a semiconductor with possible applications in spintronics.[35] A mixed EuII/EuIII oxide Eu3O4 can be produced by reducing Eu2O3 in a stream of hydrogen.[33] Neodymium and samarium also form monoxides, but these are shiny conducting solids,[25] although the existence of samarium monoxide is considered dubious.[33]

All of the lanthanides form hydroxides, Ln(OH)3. With the exception of lutetium hydroxide, which has a cubic structure, they have the hexagonal UCl3 structure.[33] The hydroxides can be precipitated from solutions of LnIII.[34] 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.[33] 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.[25]

Chalcogenides

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All of the lanthanides form Ln2Q3 (Q= S, Se, Te).[34] The sesquisulfides can be produced by reaction of the elements or (with the exception of Eu2S3) sulfidizing the oxide (Ln2O3) with H2S.[34] 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.[34] 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.[34]

Polymorphism is common amongst the sesquisulfides.[36] 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.[37] The shade of γ-Ce2S3 can be varied by doping with Na or Ca with hues ranging from dark red to yellow,[26][37] and Ce2S3 based pigments are used commercially and are seen as low toxicity substitutes for cadmium based pigments.[37]

All of the lanthanides form monochalcogenides, LnQ, (Q= S, Se, Te).[34] 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.[36] Compounds LnQ2 are known but these do not contain LnIV but are LnIII compounds containing polychalcogenide anions.[38]

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.[39] 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.[40] 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.[34]

Pnictides

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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".[26] NdN, GdN, TbN and DyN are ferromagnetic, SmN is antiferromagnetic.[41] Applications in the field of spintronics are being investigated.[35] 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.[42] 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.[34] 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.[35] The lanthanide nitrides are sensitive to air and hydrolyse producing ammonia.[24]

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.[43]

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.[44] EuC2 and to a lesser extent YbC2 hydrolyse differently producing a higher percentage of acetylene (ethyne).[45] The sesquicarbides, Ln2C3 can be formulated as Ln4(C2)3.

These compounds adopt the Pu2C3 structure[26] which has been described as having C22− anions in bisphenoid holes formed by eight near Ln neighbours.[46] The lengthening of the C-C bond is less marked in the sesquicarbides than in the dicarbides, with the exception of Ce2C3.[44] Other carbon rich stoichiometries are known for some lanthanides. Ln3C4 (Ho-Lu) containing C, C2 and C3 units;[47] Ln4C7 (Ho-Lu) contain C atoms and C3 units[48] and Ln4C5 (Gd-Ho) containing C and C2 units.[49] Metal rich carbides contain interstitial C atoms and no C2 or C3 units. These are Ln4C3 (Tb and Lu); Ln2C (Dy, Ho, Tm)[50][51] and Ln3C[26] (Sm-Lu).

Borides

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All of the lanthanides form a number of borides. The "higher" borides (LnBx where x > 12) are insulators/semiconductors whereas the lower borides are typically conducting. The lower borides have stoichiometries of LnB2, LnB4, LnB6 and LnB12.[52] Applications in the field of spintronics are being investigated.[35] The range of borides formed by the lanthanides can be compared to those formed by the transition metals. The boron rich borides are typical of the lanthanides (and groups 1–3) whereas for the transition metals tend to form metal rich, "lower" borides.[53] 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.[52]

Many methods of producing lanthanide borides have been used, amongst them are direct reaction of the elements; the reduction of Ln2O3 with boron; reduction of boron oxide, B2O3, and Ln2O3 together with carbon; reduction of metal oxide with boron carbide, B4C.[52][53][54][55] Producing high purity samples has proved to be difficult.[55] Single crystals of the higher borides have been grown in a low melting metal (e.g. Sn, Cu, Al).[52]

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.[26]

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.[55] Attempts to make EuB4 have failed.[54] The LnB4 are good conductors[52] and typically antiferromagnetic.[26]

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.[55] 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.[26][55] LaB6 and CeB6 are thermionic emitters, used, for example, in scanning electron microscopes.[56]

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.[52]

The higher boride LnB66 is known for all lanthanide metals. The composition is approximate as the compounds are non-stoichiometric.[52] 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.[52] 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.[52]

Organometallic 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].[57] Analogues of uranocene are derived from dilithiocyclooctatetraene, Li2C8H8. Organic lanthanide(II) compounds are also known, such as Cp*2Eu.[13]

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