Halides of selenium and tellurium

   In contrast to sulfur chemistry where dihalides are well established, the isolation of dihalides of selenium and tellurium has only been achieved for SeCl₂ and SeBr₂ (reactions 1.1 and 1.2). Selenium dichloride is a thermally unstable red oil; SeBr2 is a red-brown solid.

                                      (1.1)

                     (1.2)

   Table 1.1 lists selected properties of SeF₄, SeF₆, TeF₄ and TeF₆. Selenium tetrafluoride is a good fluorinating agent; it is a liquid at 298K and (compared with SF₄) is relatively convenient to handle. It is prepared by reacting SeO₂ with SF₄.

Table 1.1 Selected properties of the fluorides of selenium and tellurium.

    Combination of F₂ and Se yields SeF₆ which is thermally stable and relatively inert. The tellurium fluorides are similarly prepared, TeF4 from TeO₂ and SF₄ (or SeF₄), and TeF₆ from the elements. In the liquid and gas phases, SeF₄ contains discrete molecules (Figure 1.1a) but in the solid state, significant intermolecular interactions are present. However, these are considerably weaker than in TeF₄, in which the formation of Te^_F^_Te bridges leads to a polymeric structure in the crystal (Figure 1.1b).

Fig. 1.1 (a) The structure of SeF₄ in the gas and liquid phases; (b) in the solid state, TeF₄ consists of polymeric chains; (c) the structure of the molecular Se₄Cl₁₆-unit present in the crystal lattice of SeCl₄. Colour code: Se, yellow; Te, blue; F and Cl, green.

   Fluorine-19 NMR spectroscopic studies of liquid SeF₄ have shown that the molecules are stereochemically nonrigid. The structures of SeF₆ and TeF₆ are regular octahedra. Tellurium hexafluoride is hydrolysed by water to telluric acid, H₆TeO₆, and undergoes a number of exchange reactions such as reaction 1.3. It is also a fluoride acceptor, reacting with alkali metal fluorides and [Me₄N]F under anhydrous conditions (equation 1.4).

                              (1.3)

                (1.4)

   The[TeF₇]^-  ion has a pentagonal bipyramidal structure (1.1) although in the solid state, the equatorial F atoms deviate slightly from the mean equatorial plane. In [TeF₈]^2- , 1.2, vibrational spectroscopic data are consistent with the Te centre being in a square-antiprismatic environment.

                                                      (1.1)                                                      (1.2)

    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 Figure 1.1c for SeCl₄. 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 Cl^- ions.   A cubane contains a central cubic (or near-cubic)  arrangement of atoms. The [SeCl₃]‏ and [TeCl₃]‏ cations are also formed in reactions with Cl^- acceptors, e.g. reaction 1.5.

                                 (1.5)

   Both SeCl₄ and TeCl₄ are readily hydrolysed by water, but with group 1 metal chlorides in the presence of concentrated HCl, yellow complexes such as K₂[SeCl₆] and K₂[TeCl₆] are formed.   

   Reaction 1.6 is an alternative route to [TeCl₆]^2-, while [SeCl₆]^2- is formed when SeCl₄ is dissolved in molten SbCl₃ (equation 1.7).

                   (1.6)

                          (1.7)

     The [SeCl₆]^2- and [TeCl₆]^2- ions usually (see below) possess regular octahedral structures (Oh symmetry), rather than the distorted structure (with a stereochemically active lone pair) that would be expected on the basis of VSEPR theory. In contrast, [SeF₆]^2-has a distorted octahedral structure. On going from [SeF₆]^2-to [SeCl₆]^2-, the change from a distorted to regular octahedral structure can be attributed to a decrease in the stereochemical activity of the lone pair as the steric crowding of the ligands increases. The same trend is seen on going from [BrF₆]^- (regular octahedral) to [IF₆]^- (distorted octahedral) as the size of the central atom increases and relieves steric congestion. A word of caution, however: in the solid state, the counter-ion can influence the structure of the anion. For example, in [H₃N(CH₂)₃NH₃][TeCl₆], the [TeCl₆]^2- has approximately C_2v symmetry, and in [^tBuNH₃]₂[TeBr₆], the [TeBr₆]^2- ion has approximately C_3v symmetry. For the octahedral anions, a molecular orbital scheme can be developed (Figure 1.1) that uses only the valence shell 4p (Se) or 5p (Te) orbitals. Combined with six Cl 3p orbitals, this leads to seven occupied MOs in [ECl₆]^2- (E=Se, Te), of which four have bonding character, two have non-bonding character, and one has antibonding character. The net number of bonding MOs is therefore three, and the net E^_Cl bond order is 0.5.

Fig. 1.1 An MO diagram for octahedral [ECl₆]^2- (E=Se or Te) using a valence set of 4s and 4p orbitals for Se or 5s and 5p orbitals for Te.

   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 1.3.

(1.3)