Lanthanide dihalides

Unlike the di-f dihalides, such compounds differ little in energy from both the equivalent quantity of metal and trihalide, and from other combinations with a similar distribution of metal-metal and metal-halide bonding. So the reduced halide chemistry of the five elements shows considerable variety, and thermodynamics is ill-equipped to account for it. All four elements form di-iodides with strong metal-metal interaction, Prl2 occurring in five different crystalline forms. Lanthanum yields Lai, and for La, Ce and Pr there are hahdes M2X5 where X=Br or I. The rich variety of the chemistry of these tri-f compounds is greatly increased by the incorporahon of other elements that occupy interstitial positions in the lanthanide metal clusters [3 b, 21, 22]. 

We now turn to the 3d series elements. The dihalides and trihalides can be treated as ionic solids, although the chlorides, bromides and iodides adopt layer structures which might be better viewed as polymeric covalent crystals. In Fig. 5.2 the third ionisation energies of the 3d atoms are plotted alongside those of the lanthanides. These all involve the removal of an electron from a 3d orbital from Fe onwards, the orbital concerned is doubly occupied so that spin-pairing energy assists the ionisation. This accounts for the break between Mn and Fe, as previously discussed (Section 4.3). The increase from Sc to Mn, and from Fe to Zn, is much sharper than the corresponding increases in the lanthanide series. However, the break at the half-filled shell is less abrupt for the 3d series. This explains why the II oxidation state - which is... 

The reaction of a lanthanide metal, ytterbium, has been explored by the group of Fujiwara [164-166]. Among the various processes (coupling, desul-furisation, etc.) which were evidenced, a nice application was achieved by successive reaction of thiobenzophenone with ytterbium, and treatment of the intermediate with a dihalide. Thiolanes, for instance, were thus obtained in an expedient manner. 

Whether an individual lanthanide forms a dihalide depends npon a nnmber of factors, particnlarly the valne of h, the third ionization energy. There are three main types of lower halides. 

Another route to lanthanide dialkyls is a metathesis reaction. Lanthanide dihalides supported by a bulky monoanionic ancillary ligand, such as pentamethylcyclopentadienyl and related derivatives [4], P-diketiminato and guanidinato groups [49, 50], and so on, are generally used as the starting materials. A variety of lanthanide dialkyl compounds, including methyl compounds have been prepared and structurally characterized via successive metathesis reactions (Figure 8.11). 

Values in boldface type are from Darwent and represent his estimates of the best value and uncertainties for the energies required to break the bonds at 0 K. Where values are not available from Darwent. they arc taken from Brewer and coworkers for metal halides and dihalides (boldface italics) or from Feber for transition metal, lanthanide, and actinide halides (italics). The.se values represent enthalpies of atomization at 298 K. The remaining values are from Cottrell (Arabic numerals) and other sources (Arabic numerals with superscripts keyed to references at end of table). 

The synthesis of lanthanide and actinide compounds is the topic of a book edited by Meyer and Morss (1991). Topics that relate to halides, with the author(s) in brackets, include Lanthanide fluorides [B.G. Muller], Actinide fluorides [N.P. Freestone], Binary lanthanide(III) halides, RX3, X = Cl, Br, and I [G. Meyer], Complex lan-thanide(III) chlorides, bromides and iodides [G. Meyer], Conproportionation routes to reduced lanthanide halides [J.D. Corbett], and Action of alkali metals on lanthanide(III) halides an alternative to the conproportionation route to reduced lanthanide halides [G. Meyer and T. Schleid]. Meyer and Meyer (1992) reviewed lanthanide halides in which the valence of the lanthanide was considered unusual, with unusual being defined as compounds in which the localized valence of an atom differs from its oxidation number. A metallic halide such as Lalj [oxidation number (0)= -1-2 valence (V)= -l-3, since the 5d electron is delocalized in the conduction band] or a semiconducting halide such as PrjBtj (O = -t- 2.5 V = -I- 3) is unusual by this definition, but Tmlj (O = -1-2 V = +2) is not. In this review synthesis, properties, and calculated electronic structures are considered with emphasis on praseodymium halides and hydrogen intercalation into lanthanide dihalides and monohalides . 

There has been no evidence for the existence of a monoxide of Cf, although other compounds containing divalent Cf are known e.g., dihalides (Haire 1986). It is possible that a monoxide can be attained by high-pressure reaction of californium metal and californium oxide, as has been used for the lanthanide monoxides (Leger et al. 1980) but this reaction has not been tried with Cf. An NaCl-type lattice parameter of 0.48-0.50 would be expected for the monoxide. 

The structural chemistry of GdLiCLt and other ternary lanthanide(ni) hahdes such as NasGdCle is not the subject of this chapter, but is discussed in a handbook article. There are a few reduced ternary compoimds, however, with R = La, Ce, Pr, which have similar behavior as the respective dihalides. 

A variety of preparative methods have been described for the dihalides of Sm, Eu and Yb. Because of thermal instability of the trihalides in vacuum, Sml2, EuBr2, EUI2 and YbL are obtained by the preparative procedures usually employed for the trihalides. The commonly used methods for these lanthanides are reviewed by DeKock and Radtke (1970) and include reduction of the... 

The calculation scheme for the enthalpies of formation of lanthanide dihalides proposed by Kim and Oishi (1979) is based on the assumption that the formation of these compounds, except for europium and ytterbium, is accompanied by an electronic transition 4f 5d 6s 4f + 5d°6s in the lanthanide atom. This results in the observation of an irregularity in the variation of the thermodynamic parameters, including the enthalpies of formation, as a function of the lanthanide atomic nmnber. 

In order to obtain the enthalpy of formation for the dihalide series, the next calculation step was to sum the Avckf°(IH Ik 298) enthalpies with the value obtained from the monotonous curve that connects the enthalpies of formation for barium, europium, and ytterbium dihalides (Figure 27). It is noteworthy that Johansson (1979) proposed this calculation scheme for the enthalpy of the valence transition of the lanthanides at the same time as Kim and Oishi (1979) did. 

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