Cluster compounds lanthanide halides

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. 

The fluorite modification of RCI2, R = Sm,Eu, were also synthesized by solvolytic decomposition of reduced lanthanide halide phases with THF (Liu and Eick 1988). This modification, which had previously been reported stable only at elevated temperatures, resulted when either the vernier-type R Cl2 + 1, = 4-6, or Rj4Q33 cluster-type compound was leached. This fluorite modification has a lower density than the normal modification and transforms slowly and irreversibly to the normal modification when... 

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]. 

Very important from the chemical viewpoint are cluster halides of lanthanide metals (Ln) with the formal valence v<3 (Table 3.17). Thus, the structure and physical properties of LnE type compounds correspond in fact to the formula Ln + (I )2e, while the halides with v < 2 have typically cluster structures based on chains (single or double) of Me octahedra, surrounded by halogen atoms. The LnX type compounds constitute a peculiar class of 2-dimensional metals the Ln-Ln bond distance in them depends on the size of the anion (compare Tb-Tb distances of 3.79... 

The synthesis of compounds of the lanthanides containing cluster complexes follows in general the same routes as described in The Divalent State in Solid Rare Earth Metal Halides, the conproportionation route and the metallothermic reduction route, for example... 

---