# Lanthanide compounds

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

| 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$_{2+x}$ | EuH$_2$ o "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 "salt like" | LuH$_{2+x}$ |
| Structure | CaF$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | *PbCl$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | CaF$_2$ | | CaF$_2$ |
| metal sub lattice | fcc | fcc | fcc | fcc | fcc | fcc | o | fcc | fcc | fcc | fcc | fcc | fcc | o fcc | fcc |
| Trihydride | LaH$_{3−x}$ | CeH$_{3−x}$ | PrH$_{3−x}$ | NdH$_{3−x}$ | | SmH$_{3−x}$ | EuH$_{3−x}$ | GdH$_{3−x}$ | TbH$_{3−x}$ | DyH$_{3−x}$ | HoH$_{3−x}$ | ErH$_{3−x}$ | TmH$_{3−x}$ | | LuH$_{3−x}$ |
| 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 | 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 LnH$_2$, dihydrides. 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 where the metal sub-lattice is face centred cubic and the H atoms occupy tetrahedral sites. 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.

## Hydroxides

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 UCl$_3$ structure. The hydroxides can be precipitated from solutions of Ln$^{III}$. They can also be formed by the reaction of the sesquioxide, Ln$_2$O$_3$, with water, but although this reaction is thermodynamically favorable it is kinetically slow for the heavier members of the series. Fajans' rules indicate that the smaller Ln$^{3+}$ 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.

## Halides

Lanthanide halides :
| 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 | | CeF$_4$ | PrF$_4$ | NdF$_4$ | | | | | TbF$_4$ | DyF$_4$ | | | | | |
| Color m.p. °C | | white dec | white dec | | | | | | white dec | | | | | | |
| Structure C.N. | | UF$_4$ 8 | UF$_4$ 8 | | | | | | UF$_4$ 8 | | | | | |
| Trifluoride | LaF$_3$ | CeF$_3$ | PrF$_3$ | NdF$_3$ | PmF$_3$ | SmF$_3$ | EuF$_3$ | GdF$_3$ | TbF$_3$ | DyF$_3$ | HoF$_3$ | ErF$_3$ | TmF$_3$ | YbF$_3$ | LuF$_3$ |
| Color m.p. °C | white 1493 | 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. | LaF$_3$ 9 | LaF$_3$ 9 | LaF$_3$ 9 | LaF$_3$ 9 | LaF$_3$ 9 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 | YF$_3$ 8 |
| Trichloride | LaCl$_3$ | CeCl$_3$ | PrCl$_3$ | NdCl$_3$ | PmCl$_3$ | SmCl$_3$ | EuCl$_3$ | GdCl$_3$ | TbCl$_3$ | DyCl$_3$ | HoCl$_3$ | ErCl$_3$ | TmCl$_3$ | YbCl$_3$ | LuCl$_3$ |
| 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. | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | PuBr$_3$ 8 | PuBr$_3$ 8 | YCl$_3$ 6 | YCl$_3$ 6 | YCl$_3$ 6 | YCl$_3$ 6 | YCl$_3$ 6 |
| Tribromide | LaBr$_3$ | CeBr$_3$ | PrBr$_3$ | NdBr$_3$ | PmBr$_3$ | SmBr$_3$ | EuBr$_3$ | GdBr$_3$ | TbBr$_3$ | DyBr$_3$ | HoBr$_3$ | ErBr$_3$ | TmBr$_3$ | YbBr$_3$ | LuBr$_3$ |
| 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. | UCl$_3$ 9 | UCl$_3$ 9 | UCl$_3$ 9 | PuBr$_3$ 8 | PuBr$_3$ 8 | PuBr$_3$ 8 | PuBr$_3$ 8 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 |
| Triiodide | LaI$_3$ | CeI$_3$ | PrI$_3$ | NdI$_3$ | PmI$_3$ | SmI$_3$ | EuI$_3$ | GdI$_3$ | TbI$_3$ | DyI$_3$ | HoI$_3$ | ErI$_3$ | TmI$_3$ | YbI$_3$ | LuI$_3$ |
| 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. | PuBr$_3$ 8 | PuBr$_3$ 8 | PuBr$_3$ 8 | PuBr$_3$ 8 | | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 | BiI$_3$ 6 |
| Difluoride | | | | | | SmF$_2$ | EuF$_2$ | | | | | | TmF$_2$ | YbF$_2$ | |
| Color m.p. °C | | | | | | purple 1417 | yellow 1416 | | | | | | | grey | |
| Structure C.N. | | | | | | CaF$_2$ 8 | CaF$_2$ 8 | | | | | | | CaF$_2$ 8 | |
| Dichloride | | | | NdCl$_2$ | | SmCl$_2$ | EuCl$_2$ | | | DyCl$_2$ | | | TmCl$_2$ | YbCl$_2$ | |
| Color m.p. °C | | | | green 841 | | brown 859 | white 731 | | | black dec. | | | green 718 | green 720 | |
| Structure C.N. | | | | PbCl$_2$ 9 | | PbCl$_2$ 9 | PbCl$_2$ 9 | | | SrBr$_2$ | | | SrI$_2$ 7 | SrI$_2$ 7 | |
| Dibromide | | | | NdBr$_2$ | | SmBr$_2$ | EuBr$_2$ | | | DyBr$_2$ | | | TmBr$_2$ | YbBr$_2$ | |
| Color m.p. °C | | | | green 725 | | brown 669 | white 731 | | | black | | | green | yellow 673 | |
| Structure C.N. | | | | PbCl$_2$ 9 | | SrBr$_2$ 8 | SrBr$_2$ 8 | | | SrI$_2$ 7 | | | SrI$_2$ 7 | SrI$_2$ 7 | |
| Diiodide | LaI$_2$ metallic | CeI$_2$ metallic | PrI$_2$ metallic | NdI$_2$ high pressure metallic | | SmI$_2$ | EuI$_2$ | GdI$_2$ metallic | | DyI$_2$ | | | TmI$_2$ | YbI$_2$ | |
| 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. | CuTi$_2$ 8 | CuTi$_2$ 8 | CuTi$_2$ 8 | SrBr$_2$ 8 CuTi$_2$ 8 | | EuI$_2$ 7 | EuI$_2$ 7 | 2H-MoS$_2$ 6 | | | | | CdI$_2$ 6 | CdI$_2$ 6 | |
| Ln$_7$I$_{12}$ | La$_7$I$_{12}$ | | Pr$_7$I$_{12}$ | | | | | | Tb$_7$I$_{12}$ | | | | | | |
| Sesquichloride | La$_2$Cl$_3$ | | | | | | | Gd$_2$Cl$_3$ | Tb$_2$Cl$_3$ | | | Er$_2$Cl$_3$ | Tm$_2$Cl$_3$ | | Lu$_2$Cl$_3$ |
| Structure | | | | | | | | Gd$_2$Cl$_3$ | Gd$_2$Cl$_3$ | | | | | | |
| Sesquibromide | | | | | | | | Gd$_2$Br$_3$ | Tb$_2$Br$_3$ | | | | | | |
| Structure | | | | | | | | Gd$_2$Cl$_3$ | Gd$_2$Cl$_3$ | | | | | | |
| Monoiodide | LaI | | | | | | | | | | | | | | |
| Structure | NiAs type | | | | | | | | | | | | | | |

### Tetrahalides

Of the lanthanide tetrahalides, only the fluorides of cerium, praseodymium and terbium are well characterised.
Neodymium(IV) fluoride and dysprosium(IV) fluoride are also known under matrix conditions.

### Trihalides

All of the lanthanides form trihalides with fluorine, chlorine, bromine and iodine. They are all high melting and predominantly ionic in nature. 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.
The trihalides were important as pure metal can be prepared from them. 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 Al$_2$Cl$_6$.

### Dihalides

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, Ln$^{3+}$ (X$^−$)$_2$(e$^−$). All of the diiodides have relatively short metal-metal separations. The CuTi$_2$ structure of the lanthanum, cerium and praseodymium diiodides along with HP-NdI$_2$ contain 4$^4$ nets of metal and iodine atoms with short metal-metal bonds (393-386 La-Pr). 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 (f$^7$) and fully filled f$^{14}$. GdI$_2$ possesses the layered MoS$_2$ structure, is ferromagnetic and exhibits colossal magnetoresistance.

### Lower halides

The sesquihalides Ln$_2$X$_3$ and the Ln$_7$I$_{12}$ compounds listed in the table contain metal clusters, discrete Ln$_6$I$_{12}$ clusters in Ln$_7$I$_{12}$ and condensed clusters forming chains in the sesquihalides. Scandium forms a similar cluster compound with chlorine, Sc$_7$Cl$_{12}$ 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.
LaI is the only known monohalide. Prepared from the reaction of LaI$_3$ and La metal, it has a NiAs type structure and can be formulated La$^{3+}$ (I$^−$)(e$^−$)$_2$.

## Oxides
### Monoxides

Europium and ytterbium form salt-like monoxides, EuO and YbO, which have a rock salt structure. EuO is ferromagnetic at low temperatures, and is a semiconductor with possible applications in spintronics. A mixed Eu$^{II}$/Eu$^{III}$ oxide Eu$_3$O$_4$ can be produced by reducing Eu$_2$O$_3$ in a stream of hydrogen. Neodymium and samarium also form monoxides, but these are shiny conducting solids, although the existence of samarium monoxide is considered dubious.

### Sesquioxides

| La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu |
All of the lanthanides form sesquioxides, Ln$_2$O$_3$. The lighter (larger) lanthanides adopt a hexagonal 7-coordinate structure while the heavier/smaller ones adopt a cubic 6-coordinate "C-M$_2$O$_3$" structure. All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates. They dissolve in acids to form salts.

### Dioxides

Lanthanide dioxides, LnO$_2$, are only formed by Ce, Pr and Tb.

### Other oxides

Praseodymium and terbium also form intermediate-valence oxides of varying stoichiometry. The most stable compound of praseodymium at room temperature is Pr$_6$O$_{11}$ and the most stable of compound of terbium at room temperature is Tb$_4$O$_7$. Cerium can also form intermediate-valence oxides such as Ce$_6$O$_{11}$ and Ce$_4$O$_7$.

## Chalcogenides

All of the lanthanides form Ln$_2$Q$_3$ (Q= S, Se, Te). The sesquisulfides can be produced by reaction of the elements or (with the exception of Eu$_2$S$_3$) sulfidizing the oxide (Ln$_2$O$_3$) with H$_2$S. The sesquisulfides, Ln$_2$S$_3$ generally lose sulfur when heated and can form a range of compositions between Ln$_2$S$_3$ and Ln$_3$S$_4$. The sesquisulfides are insulators but some of the Ln$_3$S$_4$ are metallic conductors (e.g. Ce$_3$S$_4$) formulated (Ln$^{3+}$)$_3$ (S$^{2−}$)$_4$ (e$^−$), while others (e.g. Eu$_3$S$_4$ and Sm$_3$S$_4$) are semiconductors. 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.
Polymorphism is common amongst the sesquisulfides. The colors of the sesquisulfides vary metal to metal and depend on the polymorphic form. The colors of the γ-sesquisulfides are La$_2$S$_3$, white/yellow; Ce$_2$S$_3$, dark red; Pr$_2$S$_3$, green; Nd$_2$S$_3$, light green; Gd$_2$S$_3$, sand; Tb$_2$S$_3$, light yellow and Dy$_2$S$_3$, orange. The shade of γ-Ce$_2$S$_3$ can be varied by doping with Na or Ca with hues ranging from dark red to yellow, and Ce$_2$S$_3$ based pigments are used commercially and are seen as low toxicity substitutes for cadmium based pigments.
All of the lanthanides form monochalcogenides, LnQ, (Q= S, Se, Te). The majority of the monochalcogenides are conducting, indicating a formulation Ln$^{III}$Q$^{2−}$(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. Compounds LnQ$_2$ are known but these do not contain Ln$^{IV}$ but are Ln$^{III}$ compounds containing polychalcogenide anions.
Oxysulfides Ln$_2$O$_2$S 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, Gd$_2$O$_2$S doped with Tb$^{3+}$ 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.

## Pnictides
### Nitrides

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". NdN, GdN, TbN and DyN are ferromagnetic, SmN is antiferromagnetic. Applications in the field of spintronics are being investigated. CeN is unusual as it is a metallic conductor, contrasting with the other nitrides also with the other cerium pnictides. A simple description is Ce$^{4+}$N$^{3−}$ (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. 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. Alternative methods of synthesis are a high temperature reaction of lanthanide metals with ammonia or the decomposition of lanthanide amides, Ln(NH$_2$)$_3$. Achieving pure stoichiometric compounds, and crystals with low defect density has proved difficult. The lanthanide nitrides are sensitive to air and hydrolyse producing ammonia.

### Other pnictides

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 LnP$_2$, LnP$_5$, LnP$_7$, Ln$_3$As, Ln$_5$As$_3$ and LnAs$_2$.

## Carbides

Carbides of varying stoichiometries are known for the lanthanides. Non-stoichiometry is common. All of the lanthanides form LnC$_2$ and Ln$_2$C$_3$ which both contain C$_2$ units. The dicarbides with exception of EuC$_2$, are metallic conductors with the calcium carbide structure and can be formulated as Ln$^{3+}$C$_2$$^{2−}$(e–). The C-C bond length is longer than that in CaC$_2$, which contains the C$_2$$^{2−}$ anion, indicating that the antibonding orbitals of the C$_2$$^{2−}$ anion are involved in the conduction band. These dicarbides hydrolyse to form hydrogen and a mixture of hydrocarbons. EuC$_2$ and to a lesser extent YbC$_2$ hydrolyse differently producing a higher percentage of acetylene (ethyne).
The sesquicarbides, Ln$_2$C$_3$ can be formulated as Ln$_4$(C$_2$)$_3$. These compounds adopt the Pu$_2$C$_3$ structure which has been described as having C$_2$$^{2−}$ anions in bisphenoid holes formed by eight near Ln neighbours. The lengthening of the C-C bond is less marked in the sesquicarbides than in the dicarbides, with the exception of Ce$_2$C$_3$. Other carbon rich stoichiometries are known for some lanthanides. Ln$_3$C$_4$ (Ho-Lu) containing C, C$_2$ and C$_3$ units; Ln$_4$C$_7$ (Ho-Lu) contain C atoms and C$_3$ units and Ln$_4$C$_5$ (Gd-Ho) containing C and C$_2$ units. Metal rich carbides contain interstitial C atoms and no C$_2$ or C$_3$ units. These are Ln$_4$C$_3$ (Tb and Lu); Ln$_2$C (Dy, Ho, Tm) and Ln$_3$C (Sm-Lu).

## Borides
### Diborides

Diborides, LnB$_2$, have been reported for Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. All have the same, AlB$_2$, structure containing a graphitic layer of boron atoms. Low temperature ferromagnetic transitions for Tb, Dy, Ho and Er. TmB$_2$ is ferromagnetic at 7.2 K.

### Tetraborides

Tetraborides, LnB$_4$, have been reported for all of the lanthanides except EuB$_4$, all have the same UB$_4$ structure. The structure has a boron sub-lattice consists of chains of octahedral B$_6$ clusters linked by boron atoms. The unit cell decreases in size successively from LaB$_4$ to LuB$_4$. The tetraborides of the lighter lanthanides melt with decomposition to LnB$_6$. Attempts to make EuB$_4$ have failed. The LnB$_4$ are good conductors and typically antiferromagnetic.

### Hexaborides

Hexaborides, LnB$_6$, have been reported for all of the lanthanides. They all have the CaB$_6$ structure, containing B$_6$ clusters. They are non-stoichiometric due to cation defects. The hexaborides of the lighter lanthanides (La – Sm) melt without decomposition, EuB$_6$ decomposes to boron and metal and the heavier lanthanides decompose to LnB$_4$ with exception of YbB$_6$ which decomposes forming YbB$_{12}$. The stability has in part been correlated to differences in volatility between the lanthanide metals. In EuB$_6$ and YbB$_6$ 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 Ln$^{III}$ hexaborides entering conduction bands. EuB$_6$ is a semiconductor and the rest are good conductors. LaB$_6$ and CeB$_6$ are thermionic emitters, used, for example, in scanning electron microscopes.

### Dodecaborides

Lanthanide dodecaborides, LnB$_{12}$, are formed by the heavier smaller lanthanides from Gd to Lu. With the exception YbB$_{12}$ (where Yb takes an intermediate valence and is a Kondo insulator), the dodecaborides are all metallic compounds. They all have the UB$_{12}$ structure containing a 3 dimensional framework of cubooctahedral B$_{12}$ clusters.

### Higher borides

The higher boride LnB$_{66}$ is known for all lanthanide metals. The composition is approximate as the compounds are non-stoichiometric. 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 B$_{12}$ icosahedra surrounded by 12 others, B$_{12}$(B$_{12}$)$_{12}$. Other complex higher borides LnB$_{50}$ (Tb, Dy, Ho, Er, Tm, Lu) and LnB$_{25}$ are known (Gd, Tb, Dy, Ho, Er) and these contain boron icosahedra in the boron framework.

## Organolanthanide compounds

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(SiMe$_3$)$_3$]. Analogues of uranocene are derived from dilithiocyclooctatetraene, Li$_2$C$_8$H$_8$. Organic lanthanide(II) compounds are also known, such as Cp*$_2$Eu.