Thallium lower halides

The values of AH for the thallium (III) halide systems becomes less exothermic as complex formation proceeds. There are no steps with about the same value of AH , in marked contrast to e.g. Hg2+ and Pd2+. The trend of AH is in fact opposite to that found for several t)q)ical hard-hard interactions, e.g. iron (III) fluoride, lanthanum sulphate and yttrium acetate (Table 1). An even more striking feature of the thallium (III) halides is that AS°n is approximately constant for all steps. This is indeed different not only from ions such as In +, Cd2+ and Zn +, where reversals of the decreasing trend of AS°n occur for certain steps, but also from Hg2+ and Pd + where the higher steps have a much lower value of ASn than the earlier ones. 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

The first attempts (5) to reduce metal salts with sodium at low temperatures were made by researchers working with solutions of sodium in liquid ammonia. In 1925 Kraus and Kurtz (7) showed that liquid ammonia solutions of sodium could be used to reduce halides of metals that form alloys with sodium. Operating at temperatures below the boiling point of the ammonia solutions they succeeded in reducing the halides of mercury, cadmium, zinc, tin, lead, antimony, bismuth, and thallium, and, by using an excess of sodium, concomitantly produced sodium alloys of these metals. Kraus and Kurtz postulated mechanisms for the reactions and showed that many of the alloys formed were unstable in liquid ammonia— i.e., they disproportionated into free sodium and lower sodium alloys. 

For alkenyl and aryl halides, a neutral mechanistic manifold has been invoked in which one arm of the phosphine ligand must dissociate (13- 17) to create a vacant site on palladium for alkene coordination (17- 18) [6,11]. The lower enantioselectivities observed for Mizoroki-Heck reactions occurring via the neutral pathway have been attributed to this ligand dissociation. To achieve higher enantioselectivities, the reaction of alkenyl and aryl halides may be directed into the cationic manifold by the addition of the silver or thallium... 

Thallium halides fibers made of TlBr-TlI (KRS-5) are fabricated using the extrusion method [27], These fibers have a low theoretical attenuation about 6.5dB/km. However, due to material impurities and scattering the achieved attenuation is much lower, in the range of 120-350dB/km. 

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