Accounting for the Differences in the Structures and Relative Energies of the Highly Homoatomic npπ−npπ (n ≥ 3)-Bonded S2I4 2+, the Se−I π-Bonded Se2I4 2+, and Their Higher-Energy Isomers by AIM, MO, NBO, and VB Methodologies

The bonding in the highly homoatomic npπ−npπ (n ≥ 3)-bonded S2I4 2+ (three σ + two π bonds), the Se−I π-bonded Se2I4 2+ (four σ + one π bonds), and their higher-energy isomers have been studied using modern DFT and ab initio calculations and theoretical analysis methods: atoms in molecules (AIM), molecular orbital (MO), natural bond orbital (NBO), and valence bond (VB) analyses, giving their relative energies, theoretical bond orders, and atomic charges. The aim of this work was to seek theory-based answers to four main questions: (1) Are the previously proposed simple π*−π* bonding models valid for S2I4 2+ and Se2I4 2+? (2) What accounts for the difference in the structures of S2I4 2+ and Se2I4 2+? (3) Why are the classically bonded isolobal P2I4 and As2I4 structures not adopted? (4) Is the high experimentally observed S−S bond order supported by theoretical bond orders, and how does it relate to high bond orders between other heavier main group elements? The AIM analysis confirmed the high bond orders and established that the weak bonds observed in S2I4 2+ and Se2I4 2+ are real and the bonding in these cations is covalent in nature. The full MO analysis confirmed that S2I4 2+ contains three σ and two π bonds, that the positive charge is essentially equally distributed over all atoms, that the bonding between S2 and two I2 + units in S2I4 2+ is best described by two mutually perpendicular 4c2e π*−π* bonds, and that in Se2I4 2+, two SeI2 + moieties are joined by a 6c2e π*−π* bond, both in agreement with previously suggested models. The VB treatment provided a complementary approach to MO analysis and provided insight how the formation of the weak bonds affects the other bonds. The NBO analysis and the calculated AIM charges showed that the minimization of the electrostatic repulsion between EI2 + units (E = S, Se) and the delocalization of the positive charge are the main factors that explain why the nonclassical structures are favored for S2I4 2+ and Se2I4 2+. The difference in the structures of S2I4 2+ and Se2I4 2+ is related to the high strength of the S−S π bond compared to the weak S−I σ bond and the additional stabilization from increased delocalization of positive charge in the structure of S2I4 2+ compared to the structure of Se2I4 2+. The investigation of the E2X4 2+ series (E = S, Se, Te; X = Cl, Br, I) revealed that only S2I4 2+ adopts the highly npπ−npπ (n ≥ 3)-bonded structure, while all other dications favor the π-bonded Se2I4 2+ structure.