Halides of selenium and tellurium

In contrast to sulfur chemistry where dUialides are well established, the isolation of dUialides of selenium and tellurium has only been achieved for SeCl2 and SeBr2 (reactions 16.75 and 16.76). Selenium dichloride is a thermally unstable red oil SeBr2 is a red-brown solid. 

Physical appearance and general characteristics Colourless fuming liquid toxic violent hydrolysis White solid at low temp. colourless gas toxic Colourless solid highly toxic White solid at low temp. colourless gas foul smelling highly toxic 

Tellurium hexafluoride is hydrolysed by water to telluric acid, HgTeOg (see 16.62), and undergoes a number of exchange reactions such as reaction 16.77. It is also a fluoride acceptor, reacting with alkali metal fluorides and [Me4N]F under anhydrous conditions (eq. 16.78). 

14 (a) The structure of Sep4 in the gas and liquid phases, (b) In the solid state, Tep4 consists of polymeric chains the 

Te-F—Te bridges are asymmetrical (Te-F = 208 and 228 pm), (c) The structure of the molecular Se4Clis-unit present in crystalline SeCb. Colour code Se, yellow Te, blue F and Cl, green. 

In contrast to S, Se and Te form stable tetrachlorides, made by direct combination of the elements. Both the tetrachlorides are solids (SeCl4, colourless, subl. 469 K TeCl4 yellow, mp 497 K, bp 653 K) which contain tetrameric units, depicted in Pigure 15.13c for SeC. The E—Cl (E = Se or Te) bonds within the cubane core are significantly longer than the terminal E—Cl bonds e.g. Te—Cl = 293 (core) and 231 (terminal) pm. Thus, the structure may also be described in terms of [ECl3] and d ions. 

A cubane contains a central cubic (or near-cubic) arrangement of atoms. 

The [SeCl3]+ and [TeCl3] cations are also formed in reactions with d acceptors, e.g. reaction 15.75. 

Tellurium forms a series of subhalides, e.g. Te3Cl2 and Te2Cl, the structures of which can be related to the helical chains in elemental Te. When Te is oxidized to Te3Cl2, oxidation of one in three Te atoms occurs to give polymer 15.37. 

The [TeFv] ion has a pentagonal bipyramidal structure (16.39) although in the solid state, the equatorial F atoms deviate slightly from the mean equatorial plane. In [TeFg], 16.40, vibrational spectroscopic data are consistent with the Te centre being in a square-antiprismatic environment. 

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In the series of the binary halides of selenium and tellurium, the crystal structure determinations of tellurium tetrafluoride (100) and of tellurium tetrachloride on twinned crystals (65, 66) were the key to understanding the various and partly contradictory spectroscopic and other macroscopic properties (e.g., 66,161,168,169,219,220, 412), as well as the synthetic potential of the compounds. In contrast to the monomeric molecular i//-tbp gas phase structures with C2v symmetry (417), the solid state structures of both are polynuclear. As the prototype of the chlorides and bromides of selenium and tellurium, crystalline tellurium(IV) chloride has a cubane-like tetrameric structure with approximate Td symmetry (Fig. 1). Within the distorted TeCla+a octa-hedra the bonds to the triply bridging chlorine ligands are much longer than to the terminal chlorines. The bonding system can be described either covalently as Te4Cli6 molecules, or, in an ionic approximation, as [(TeCl Cn4] with a certain degree of stereochemical activity of the lone pairs toward the center of the voluminous cubane center (65, 66). 

A variety of halides of selenium and tellurium has been synthesized, but their somewhat specialized chemistries and structures are not considered. 

Despite the tremendous amount of work on the binary compounds, copper chalcogenide halides were first reported in 1969 (304). Nine compounds of selenium and tellurium have been found, and they are listed in Table 1. Copper sulfide halides are still unknown. 

Nitrogen and sodium do not react at any temperature under ordinary circumstances, but are reported to form the nitride or azide under the influence of an electric discharge (14,35). Sodium siHcide, NaSi, has been synthesized from the elements (36,37). When heated together, sodium and phosphoms form sodium phosphide, but in the presence of air with ignition sodium phosphate is formed. Sulfur, selenium, and tellurium form the sulfide, selenide, and teUuride, respectively. In vapor phase, sodium forms haHdes with all halogens (14). At room temperature, chlorine and bromine react rapidly with thin films of sodium (38), whereas fluorine and sodium ignite. Molten sodium ignites in chlorine and bums to sodium chloride (see Sodium COMPOUNDS, SODIUM HALIDES). 

The first compound of this series, CeSI, was reported by Carter (68) in 1961, and later discussed by Dagron (93). It was obtained by the reaction of iodine with cerium sulfide at 430 C, or by direct synthesis from the elements at 500°C. This was the start of a detailed investigation of this group of compounds mainly by Dagron and co-workers. The present situation is presented in Table VII. No scandium compounds are known thus far, and the same is true for selenium and tellurium halides of these elements. 

If you move left one column in the periodic table from the halides, the chalcogenides need two electrons to complete their valence shell, and thus can bond to the surface and each other simultaneously. This appears to account for much of the interesting surface chemistry of chalcogenide atomic layers. Chalcogenides, including oxides (corrosion), are some of the most studied systems in surface chemistry. The oxides are clearly the most important, but significant amounts of work have been done with sulfur, selenium and tellurium. 

Inorganic Compounds. Inorganic selenium compounds are similar to those of sulfur and tellurium. The most important inoiganic compounds are the selenides, halides, oxides, and oxyacids. Selenium oxidation states are —2, 0, +1, +2, +4, and H-6. Detailed descriptions of the compounds, techniques, and methods of preparation, and references to original work are available (1—3,5,6—10, 51—54). Some important physical properties of inorganic selenium compounds are Us ted in Table 3. 

The other halides of boron and silicon, and also of, phosphorus, sulphur, selenium, tellurium, and iodine, react at once with water, forming hydrogen halide and an acid. The equations are as follows —... 

Similar trans effect correlations have also been observed for related selenium and tellurium halide complexes by Knop and coworkers and by Krebs and Ahlers (19) a number of structural similarities between group 16 halides and halogenoanions are evident and these are mentioned where appropriate in the following section. 

Selenium and Tellurium Halides. For selenium, these halides are mostly of marginal stability and the characterization is incomplete the most stable ones are Se2Cl2 and Se2Br2, whose structures are known. In acetonitrile there are the following equilibria ... 

Analogous sulfur, selenium, and tellurium compounds are easily prepared by reaction of the aluminum halides with the appropriate aluminum chalcogenide (equation 35). 

In addition to complex halides and cyanides, the metals form many ammines. Organic compounds of nitrogen, arsenic, sulphur, selenium and tellurium arc other possible ligands. Many complexes with bidentate ligands are known. The various types of isomerism are common thus Pd(NH3)2Cl2 occurs as the pink [Pd (NH3)4] [PdCy and as the yellow cis and trans isomers ... 

Silastannathianes containing the unit SiSSn can be prepared from the reaction of a mixture of tin halide and silicon halide with hydrogen sulfide in the presence of an amine, and the corresponding selenium and tellurium compounds from a similar reaction using lithium selenide or lithium telluride.14... 

Since PPS is prepared by reaction of p-dichlorobenzene (I) and Na2S in N-methyl-2-pyrrolidone (NMP) at ca, 240° (13), the reaction of p-dlhalobenzenes with Na Se and Na2Te appeared to be at attractive approach. We note that an unsuccessful attempt to synthesize PPSe from p-dibromobenzene (2) and Na2Se has been reported. (14) As we have recently found (la-b) that alkali metals react directly with selenium and tellurium in 1 1 and 2 1 atomic ratios in dipolar aprotic solvents, thus avoiding the use of liquid NH3 commonly used for the preparation of these reagents, and that these reagents react readily with aromatic halides, which are not activated in the usual sense for a nucleophilic... 

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