Halides dihalides, properties

The hypothesis of the formation of the HC12" ion agrees with the properties which TMS shows in favoring the formation of large hydrogen dihalide ions rather than small halide ions, as found by Benoit and coworkers (32,33). Moreover, acid-anion complexes also were observed... 

All of the alkaline earth metals form saltlike MX2 dihahdes (see Alkaline Earth Metals Inorganic Chemistry). Within this group, we observe stmctural effects owing to both cation sizes and properties of the halide ions. From Be to Ba, coordination numbers monotonically increase from four (tetrahedral) in Be dihalides to eight (cubic) or nine (tricapped trigonal prismatic) in the Ba systems. 

Compounds of divalent Ge are well known. There is no evidence that GeO is a stable phase at temperatures below 1000°K (compare SiO), but compounds stable at ordinary temperatures include GeS, all four dihalides, and complex halides such as MGeCla. The Ge ion is not stable in water, and its crystal structure shows that GeF2 is not a simple ionic crystal (p. 929). There seem to be no simple ionic crystalline stannous compounds. In solution Sn presumably exists as complexes its easy conversion into Sn" gives stannous compounds their reducing properties. Lead presents a quite different picture. The stable ion is Pb . This has no reducing properties, and there is no evidence that Pb can exist in aqueous solution, but it certainly exists in crystalline compounds such as Pb02 (rutile structure). 

The experimental and theoretical studies on alkali halides are discussed and summarized in a book edited by Davidovits and McFadden [434]. Molecular parameters and thermodynamic properties including partial pressures are given for alkali halides and metal dihalides by Brewer and Brackett [43 S] as well as Brewer et al. [436], respectively. 

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 . 

A particularly active area of halide chemistry has been the investigation of the reduced halide systems. Dihalides of Sm, Eu and Yb have been known for years, but thorough examination of their properties has only recently been undertaken. Some of the most interesting reduced halides have been observed in the metal-rich chloride, bromide and iodide systems of normally trivalent rare earths. Numerous intermediate phases, RX ,(2solid state. The gaseous dihalides... 

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