s: clockwise from top center:
praseodymium,
cerium,
lanthanum,
neodymium,
samarium and
gadolinium The chemistry of the lanthanides is dominated by the +3 oxidation state, and in LnIII compounds the 6s electrons and (usually) one 4f electron are lost and the ions have the configuration [Xe]4f(
n−1). 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 Ln0/3+ 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 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). : 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 Lanthanide metals react exothermically with hydrogen to form LnH2, dihydrides. 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 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. They dissolve in acids to form salts. A mixed EuII/EuIII 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. ==Physical properties==