Oxides of selenium and tellurium

   Selenium and tellurium dioxides are white solids obtained by direct combination of the elements. The polymorph of TeO₂ so formed is α-TeO₂, whereas β-TeO₂ occurs naturally as the mineral tellurite. Both forms of TeO2 contain structural units 1.1 which are connected by shared O atoms into a three-dimensional lattice in α-TeO₂, and into a sheet structure in the β-form. The structure of SeO₂ consists of chains (1.2) in which the Se centres are in trigonal pyramidal environments. Whereas SeO₂ sublimes at 588 K, TeO₂ is an involatile solid (mp 1006 K). In the gas phase, SeO₂ is monomeric with structure 1.3. The trends in structures of the dioxides of S, Se and Te and their associated properties (e.g. mp, volatility) reflect the increase in metallic character on descending group 16.

                                                       (1.1)                               (1.2)                                   (1.3)

   Selenium dioxide is very toxic and is readily soluble in water to give selenous acid, H₂SeO₃. It is readily reduced, e.g. by hydrazine, and is used as an oxidizing agent in organic reactions. The α-form of TeO₂ is sparingly soluble in water, giving H₂TeO₃, but is soluble in aqueous HCl and alkali. Like SeO₂, TeO₂ is a good oxidizing agent. Like SO₂, SeO₂ and TeO₂ react with KF (see equation 15.85). In solid K[SeO₂F], weak fluoride bridges link the [SeO₂F]^- ions into chains. In contrast, the tellurium analogue contains trimeric anions (structure 1.4, see worked example below). Selenium trioxide is a white, hygroscopic solid. It is difficult to prepare, being thermodynamically unstable with respect to SeO₂ and O₂ (Δ_fH^o(298 K): SeO₂ = ^_225; SeO₃ = ^_184 kJ mol^-1). It may be made by reaction of SO₃ with K₂SeO₄ (a salt of selenic acid). Selenium trioxide decomposes at 438 K, is soluble in water, and is a stronger oxidizing agent than SO₃. In the solid state, tetramers (1.5) are present.

(1.4)                                  (1.5)

    Tellurium trioxide (the a-form) is formed by dehydrating telluric acid (equation 1.1). It is an orange solid which is insoluble in water but dissolves in aqueous alkali, and is a very powerful oxidizing agent. On heating above 670 K, TeO₃ decomposes to TeO₂ and O₂. The solid state structure of TeO₃ is a three-dimensional lattice in which each Te(VI) centre is octahedrally sited and connected by bridging O atoms.

                                                    (1.1)

Worked example

Selenium and tellurium oxides and their derivatives Diagram 1.4 shows a representation of the structure of [Te₃O₆F₃]^3- . The coordination environment of the Te atom is not tetrahedral. Rationalize this observation.

Apply VSEPR theory to structure 1.4:   Te is in group 16 and has six valence electrons.  The formation of Te^_F and three Te^_O bonds (terminal and two bridging O atoms) adds four more electrons to the valence shell of Te.    In [Te₃O₆F₃]^3- , each Te centre is surrounded by five electron pairs, of which one is a lone pair. Within VSEPR theory, a trigonal bipyramidal coordination environment is expected.