Cerium Compounds

Cerium compounds are compounds containing the element cerium (Ce), a lanthanide. Cerium exists in two main oxidation states, Ce(III) and Ce(IV). This pair of adjacent oxidation states dominates several aspects of the chemistry of this element. Cerium(IV) aqueous solutions may be prepared by reacting cerium(III) solutions with the strong oxidizing agents peroxodisulfate or bismuthate. The value of ''E''^⦵(Ce⁴⁺/Ce³⁺) varies widely depending on conditions due to the relative ease of complexation and hydrolysis with various anions, although +1.72 V is representative. Cerium is the only lanthanide which has important aqueous and coordination chemistry in the +4 oxidation state.Greenwood and Earnshaw, pp. 1244–8

Binary compounds

Halides

Cerium forms all four trihalides CeX₃ (X = F, Cl, Br, I) usually by reaction of the oxides with the hydrogen halides. The anhydrous halides are pale-colored, paramagnetic, hygroscopic solids. Upon hydration, the trihalides convert to complexes containing aquo complexes e(H₂O)_8-9sup>3+. Unlike most lanthanides, Ce forms a tetrafluoride, a white solid. It also forms a bronze-colored diiodide, which has metallic properties.Greenwood and Earnshaw, pp. 1240–2

Aside from the binary halide phases, a number of anionic halide complexes are known. Fluoride gives the Ce(IV) derivatives and . Chloride gives the orange .

Oxides and chalcogenides

Cerium(IV) oxide ("ceria") has the fluorite structure, similarly to the dioxides of praseodymium and terbium. Ceria is a nonstoichiometric compound, meaning that the real formula is CeO_2−x, where x is about 0.2. Thus, the material is not perfectly described as Ce(IV). Ceria reduces to cerium(III) oxide with hydrogen gas.

Many nonstoichiometric chalcogenides are also known, along with the trivalent Ce₂Z₃ (Z = S, Se, Te). The monochalcogenides CeZ conduct electricity and would better be formulated as Ce³⁺Z²⁻e⁻. While CeZ₂ compounds are known, they are polychalcogenides with cerium(III): cerium(IV) derivatives of S, Se, and Te are unknown.Greenwood and Earnshaw, pp. 1238–9

Other binary compounds

Cerium phosphide is a gray crystal, which can be obtained by reacting cerium dioxide and phosphine at 1300 °C in the presence of hydrogen,Vasil'eva, I. G.; Mironov, K. E.; Mironov, Yu. I. Properties of cerium monophosphide. ''Redkozemel. Metal. Ikh Soedin., Mater. Vses. Simp.'', 1970: 160-165. . or by reacting sodium phosphide and cerium chloride at 700~800 °C.

Cerium(IV) complexes

The compound ceric ammonium nitrate ("CAN") is the most common cerium compound encountered in the laboratory. The six nitrate ligands bind as bidentate ligands. The complex is 12-coordinate, a high coordination number which emphasizes the large size of the Ce⁴⁺ ion. CAN is popular oxidant in organic synthesis, both as a stoichiometric reagent and as a catalyst. It is inexpensive, easily handled. It operates by one-electron redox. Cerium nitrates also form 4:3 and 1:1 complexes with 18-crown-6 (the ratio referring to that between cerium and the crown ether).

Classically CAN is a primary standard for quantitative analysis. Cerium(IV) salts, especially cerium(IV) sulfate, are often used as standard reagents for volumetric analysis in cerimetric titrations.

Cerium(III) and terbium(III) have ultraviolet absorption bands of relatively high intensity compared with the other lanthanides, as their configurations (one electron more than an empty or half-filled f-subshell respectively) make it easier for the extra f electron to undergo f→d transitions instead of the forbidden f→f transitions of the other lanthanides.Greenwood and Earnshaw, pp. 1242–4 Cerium(III) sulfate is one of the few salts whose solubility in water decreases with rising temperature.

Due to ligand-to-metal charge transfer, aqueous cerium(IV) ions are orange-yellow. Aqueous cerium(IV) is metastable in water and is a strong oxidizing agent that oxidizes hydrochloric acid to give chlorine gas.

In the Belousov–Zhabotinsky reaction, cerium oscillates between the +4 and +3 oxidation states to catalyze the reaction.

Organocerium compounds

Organocerium chemistry is similar to that of the other lanthanides, often involving complexes of cyclopentadienyl and cyclooctatetraenyl ligands. Cerocene (Ce(C₈H₈)₂) adopts the uranocene molecular structure.Greenwood and Earnshaw, pp. 1248–9 The 4f electron in cerocene, , is poised ambiguously between being localized and delocalized and this compound is also considered intermediate-valent.
Alkyl, alkynyl, and alkenyl organocerium derivatives are prepared from the transmetallation of the respective organolithium or Grignard reagents, and are more nucleophilic but less basic than their precursors.

See also

* Praseodymium compounds, Terbium compounds (other lanthanide compounds that can form in the +4 oxidation state)
* Thorium compounds (actinide analogue)

References

{{Cerium compounds

Cerium
Cerium compounds
Chemical compounds by element