The elusive diiodosulphanes and diiodoselenanes

Diiodosulphanes (SnI2, where 1 ≤ n ≤ 6) are scarcely studied. There are indirect evidences of the existence of diiodoselenanes (SenX2) such as RMN or UV–vis spectroscopy. The geometry of these compounds was stated at different theory levels: DFT (M06-2X-D3, PBE0) and MP2. Certain S–S and Se–Se bonds have bond orders higher than 1, and the bond among the S or Se atoms has some pi character. These compounds are involved in many reactions in equilibrium. The thermochemistry of these reactions was studied using DFT, MP2, MP4(SDTQ) and, in some cases, with CCSD(T) or CCSD(TQ) methods. For reactions such as SmI2 + SnI2 → Sm+nI2 + I2, the used methods stated that ΔH is around −25 to −50 kJ/mol when the length of the S–S chain increases. In these reactions, the equilibrium is displaced towards the formation of products. Instead, when the reactants are diiodoselenanes, ΔH goes from −4 to −23 kJ/mol. Finally, the electronic spectra of these compounds were described using TDDFT methods. The results for diiodosulphanes matched with the experimental data, which allowed for the assignment of the different transitions. The electronic spectra for diiodoselenanes, which are unknown for the moment, were estimated.