Tetrahalides trihalides

N-Bridged tellurium-containing heterocycles are exemplified by derivatives of l,4-tellurazine[2,3-g/i]carbazole 107 and heterocycle 108. One-pot reaction of A-propargylcarbazole with tellurium tetrahalides under conditions of the two-phase tellurohalogenation (Section n,B,l) leads to halogenomethylene derivatives 107 (90KGS126). The reaction proceeds as the spontaneous electrophilic cycliza-tion of initially formed tellurium trihalides 109. 

By addition of tellurium tetrahalides and aryltellurium trihalides to acetylenes... 

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

The dihalides of titanium, formed by reduction of the tetrahalides, are vigorous reducing agents and unstable T1CI2 is inflammable in air. The trihalides, though more stable than the dihalides, are effective reducing agents. Ti(TTT) occurs in aqueous solutions as Ti(1120)( 3, ... 

Chlorofluoro compounds were electroreductivelv added to aldehydes to give the corresponding fluorinated alcohols (231) (equation 124)214. In this process platinum or lead are employed as the cathode, under constant-current conditions. The starting dihalides or trihalides (halogen = F, Cl or Br) can be prepared on an industrial scale by the reaction of the corresponding tri- or tetrahalide at a sacrificial anode (Zn, A1 or Mg)220. 

Triphenyl telluronium halides reacted with tellurium tetrahalides or phenyl tellurium trihalides to produce telluronium salts with tellurium in the cation as well as in the anion8. 

The major types are the trihalides (EX3), the pentahalides (EX5), and the dielement tetrahalides (E2X4). We shall discuss them in that order. All trihalides except PF3 are best obtained by direct halogenation, keeping the element in excess, whereas the pentahalides may all be prepared by reaction of the elements with excess halogen. Special methods are used for E2X4 compounds. 

The compounds known are summarized in Table 10.1. The only compound of an early actinide in the -1-2 state is Thl2, a metallic conductor which is probably Th + (e )2 (D)2-Certain heavier actinides form MX2 (Am, Cf, Es), which usually have the structure of the corresponding EuX2 and are thus genuine M + compounds. All four trihalides exist for all the actinides as far as Es, except for thorium and protactinium. Tetrafluorides exist for Th-Cm and the other tetrahalides as far as NpX4 (and in the gas phase in the case of PuCE). Pentahalides are only known for Pa, U, and Np whilst there are a few MFe (M = U-Pu), uranium is the only actinide to form a hexachloride. The known actinide halides are generally stable compounds most are soluble in (and hydrolysed by) water. 

The development of the chemistry of niobium and tantalum in their lower oxidation states, three or less, has long been precluded by the lack of convenient starting materials. Indeed, stoichiometric trihalides MX3 (X = Cl, Br) are only incidental compositions. They are obtained by reduction of the pentahalides or disproportionation of the tetrahalides, and are polymeric and rather inert. 

During the search for homogeneous systems in which the tetravalent perhalotellurium compounds would undergo addition to an alkene, trimethylchlorosilane, acetyl chloride, or acetyl bromide were found to be a convenient source of halogens for the conversion of tellurium dioxide to a tellurium tetrahalide88. Addition of twice the stoichiometrically required amount of cyclohexene to a solution of tellurium tetrahalide, thus generated in either acetic acid, dichloromethane or ethanol-free chloroform leads to the formation of (fra i-2-halocyclo-hexyl)tellurium trihalides in ca. 70% yield. When dry methanol is used as solvent, trans-2-... 

Diaryl ditellurium compounds and v/c-dibromides, aryl tellurium trihalides diaryl tellurium dihalides, tellurium tetrahalides or sulfur tetrafluoride react to give diaryl tellurium dihalides and elemental tellurium. All of these reactions might proceed via aryl tellurium halides that subsequently disproportionate to the observed products ... 

Table9 Organo Tellurium Trihalides from Equimolar Amounts of Tellurium Tetrahalides and Main Group Organometallic Compounds... 

Hexaorganoantimony and -bismuth anions have been described in Section III.A. Monoorganoantimony tetrahalides and diorganoantimony trihalides react readily with halide anions to form the corresponding hexacoordinate anions. Several examples are shown in equations 288 289 ", 290 and 29P" ... 

The interatomic distance is estimated from that of the solid, given as 2.46 A by Natta et al. (4), and from comparisons of bond distances of crystal and gas phase tetrahalides of titanium. The pyramidal bond angle is estimated assuming TiClg(g) similar to the group V trihalides. The principal moments of inertia are B 42.8614 x 10" and I = 73.1025 x 10 g cm. ... 

With the exception of thorium, the actinides form trihalides. For uranium and neptunium, reduction of the MX4 compounds with hydrogen is necessary, but for the elements from plutonium onwards the action of the carbon tetrahalide or aluminium halide on the dioxide is usually employed. The trifluorides are insoluble but the rest dissolve to give solutions containing ions. 

Halides of uranium. Our present knowledge of the structures of the majority of the 5f halides is summarized (with references) under Trihalides, Tetrahalides, etc. in Chapter 9. The halides of U include ... 

NF3 is the most stable of the trihalides of nitrogen, being the only one to have a negative value of (Table 14.5). It is a colourless gas which is resistant to attack by acids and alkalis, but is decomposed by sparking with H2 (equation 14.54). The resistance towards hydrolysis parallels that observed for the carbon tetrahalides Section 13.8). 

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