The scant attention paid to tellurium in both inorganic and organic chemistry textbooks may reflect, in part, the very low natural abundance of the element. Such treatments commonly imply that the structures and reactivities of tellurium compounds can be extrapolated from the behaviour of their lighter chalcogen analogues (sulfur and selenium). In fact, recent findings and well-established observations clearly illustrate that this assumption is not valid. The emerging importance of the unique properties of tellurium compounds is apparent from the variety of their known and potential applications in both inorganic and organic chemistry, as well as materials science. With reference to selected contemporary examples, this Tutorial Review examines the fundamental concepts that are essential for an understanding of the unique features of tellurium chemistry with an emphasis on hypervalency, three-centre bonding, secondary bonding interactions, σ and π-bond energies (multiply bonded compounds), and Lewis acid behaviour.

The hydrolysis reactions of AlCl3 in 0.1 M aqueous solutions at pH 3.27-4.20 were monitored by electrospray ionization time-of-flight mass spectrometry (ESI-ToF MS) as a function of time. The cationic and anionic ESI mass spectra of aluminum(III) solutions gave strong evidence of the presence of a variety of monomeric and polymeric complexes. Competition between the OH- and Cl- ligands within the same aluminum core was observed. The influence of the sample cone voltage on the product distribution was also explored. The optimum sample cone voltage for the cationic spectra was 70 V. For the anionic spectra no optimum sample cone voltage was found and the appearance of the anionic spectra was strongly dependent on the sample cone voltage within the whole range explored.

A simple and efficient synthesis of solutions of pure SeCl2 in THF or dioxane (ca. 0.4 M) at 23 °C was achieved by treatment of elemental selenium with an equimolar amount of SO2Cl2. SeCl2 was characterized by 77Se NMR and Raman spectra. SeCl2 forms 1:1 or 1:2 adducts with tetramethylthiourea (tmtu) or tetrahydrothiophene (tht), respectively. The crystal structure of SeCl2·tmtu (1) reveals a T-shaped geometry [d(Se−Cl) = 2.443(4) Å] with weak intramolecular Se···Cl interactions [d(Se−Cl) = 3.276(4) Å]. Crystals of 1 are triclinic, space group P1̄, with a = 8.473(3) Å, b = 9.236(3) Å, c = 7.709(4) Å, α = 109.90(3)°, β = 92.26(4)°, γ = 107.89(3)°, V = 532.9(4) Å3, and Z = 2. The complex SeCl2(tht)2 (2) adopts a square planar geometry with d(Se−Cl) = 2.4149(8) Å. Crystals of 2 are monoclinic, space group C2/c, with a = 15.6784(8) Å, b = 9.1678(4) Å, c = 9.1246(4) Å, β = 110.892(2)°, V = 1225.3(1) Å, and Z = 4. The reaction of Ph3PS with SeCl2 gives Ph3PCl2 and a complex mixture of selenium sulfides Se n S8 - n (n = 1−5), which were identified by 77Se NMR. Halogen exchange between SeCl2 and Me3SiBr in THF yields thermally unstable SeBr2 (ca. 0.4 M) characterized by 77Se NMR and Raman spectra.

The electronic structures and molecular properties of square-planar 6-electron ring molecules and ions E2N2 and E4 2+ (E = S, Se, Te) were studied using various ab initio methods and density functionals. All species were found to contain singlet diradical character in their electronic structures. Detailed analysis of the CAS wave function of S2N2 in terms of different valence bond structures gives largest weight for a Lewis-type singlet diradical VB structure in which the two unpaired electrons reside on nitrogen atoms, though the relative importance of the different VB structures is highly dependent on the level of theory. The diradical character in both E2N2 and E4 2+ was found to increase in the series S < Se < Te. The diradical nature of the chemical species is manifested in the prediction of molecular properties, in which the coupled cluster and multiconfigurational approaches, as well as the BPW91 functional show consistent performance. 77 Se NMR chemical shifts of chalcogen cations SxSe4-x 2+ (x = 0-3) were calculated with CAS, BPW91 and B3PW91 methods using the GIAO formalism. The hybrid functional B3PW91 shows inferior performance, but both CAS and BPW91 unquestionably confirm the experimental assignment and are able to predict the NMR chemical shifts of these computationally difficult cases with excellent accuracy.