Lanthanide halides stability

In particular the redox stability of lanthanide cations makes the route superior to, e.g., the usual metathesis reaction involving lanthanide halides. The advantages are evident ... 

Additional areas of lanthanide halide chemistry that have been reviewed include the synthesis, phase studies, and structures of complex lanthanide halides - compounds formed between one or more group 1 cation and lanthanide element halides (Meyer 1982). Halides in combination with lanthanide elements in the II, III, and IV oxidation states were considered with the chemistry of the heavier halides being emphasized. More recently the reduced ternary lanthanide halides (Meyer 1983) and the reduced halides of the lanthanide elements were reviewed (Meyer 1988). The latter review considered lanthanides in which the formal oxidation state of the cation was 2 and included hydride halides, oxide halides, mixed-valence ternary halides, and reduced halide clusters. Corbett et al. (1987) discussed the structures and some bonding aspects of highly reduced lanthanide halides and compounds stabilized by a second-period element bound within each cluster, e.g., SC7CIJ2B, SC5CI5B, YjCljC. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

Acetyl ligands, in niobium complexes, C-H BDEs, 1, 298 Achiral phosphines, on polymer-supported peptides, 12, 698 Acid halides, indium compound reactions, 9, 683 Acidity, one-electron oxidized metal hydrides, 1, 294 Acid leaching, in organometallic stability studies, 12, 612 Acid-platinum rf-monoalkynes, interactions, 8, 641 Acrylate, polymerization with aluminum catalysts, 3, 280 Acrylic monomers, lanthanide-catalyzed polymerization,... 

As noted earlier, pentamethyl substitution in the cyclopentadienyl ligand confers some stability allowing ligand redistribution and the synthesis of chloride complexes of lanthanides. Stabilization also occurs by coordination with Lewis bases or alkali halides. 

Many studies on the absorption spectra of lanthanides in alcoholic media have been made and the observations and anomalies have been explained in terms of entry of a chloride ion into the coordination sphere of the lanthanide ion. The composition and stability of halide complexes of lanthanides in alcohol and aqueous alcoholic solutions have been studied by spectral techniques. The halide ions have been found to cause marked changes in the spectra of lanthanides in alcoholic and aqueous media. The observed spectral changes may be attributed to changes in the immediate coordination environment of the lanthanide ion [223]. 

The halides are all typically ionic solids, but can be vaporized as molecules, the structures of which are not all linear. On account of its dispersion and transparency properties, CaF2 is used for prisms in spectrometers and for cell windows (especially for aqueous solutions). It is also used to provide a stabilizing lattice for trapping lanthanide +2 ions. 

Several routes are currently applied to synthesize cationic organolanthanide species, including the halide abstraction from heteroleptic Ln(III) compounds [Eq. (25)] [152], the oxidation of divalent metallocenes [Eqs. (26) and (27)] [153], the protolysis of lanthanide alkyl and amide moieties [Eqs. (28) and (29)] [154,155], and anion exchange [Eqs. (30) and (31)] [84,156]. In the absence of a coordinating solvent such extremely electrophilic species attain stabilization via arene interactions with the BPh4 anion (Sect.5.1) [153b]. Cationic rare earth species have been considered as promising candidates for Lewis acid catalysis [157]. 

One of the most interesting applications of the HSAB concept consists in the prediction of the stability of the complexes formed owing to interaction of alkali metal halides with rare-earth metal halides. These systems are of great interest for the materials science of scintillation materials the said complex halides are now considered among the most promising scintillation detectors and sensors. Besides, the Li- and Gd-based materials are especially convenient as effective detectors of thermal neutrons. The compositions and stability of the formed compounds depend considerably on the kind of acids and bases from which the compound is formed. So, Li+ cation is one of the hardest cation acids, and, therefore, the formation of stable complex halides of Li and lanthanides according to reaction ... 

Oj to 02, which is again a measure of the ability to stabilize anionic charge. The fluorescence shifts are given in Table 3.31 and the correlation is shown in Figure 3.32. The orders Li >Na+ andMg +>Ca +>Sr > Ba + show that Lewis acidity decreases with the size of the cation. According to this scale, the tin halides are comparable in acidity to Mg +. The strong Lewis acidity of Sc +, and the lanthanides, such as La + and Yb, has been exploited in various synthetic applications. 

The organometallic chemistry of the rare earths deals mainly with bis (cyclopentadienyl) derivatives due to the easy available bis (cyclopentadienyl) rare earth chlorides and other halides via reaction of the rare earth trichlorides with two equivalents of a cyclopentadienyl alkali salt. Bis(cyclopentadienyl)lanthanide chlorides are formed as chloride-bridged dimers (Figure 28a), as monomers stabilized by a donor molecule like THF (Figure 28b) or as afecomplexes with alkali halides (Figure 28c). 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

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